DEPARTMENT OF COMMERCE 



Technologic Papers 

OF THE 

Bureau of Standards 

S. W. STRATTON. Director 



No. 59 

STANDARD TEST SPECIMENS OF ZINC BRONZE 

( Cu 88, Sn 10, Zn 2 )— PARTS I AND H 
PART I.— PREPARATION OF SPECIFICATIONS 

BY 

C. P. KARR, Associate Physicist 
PART II.— MICROSTRUCTURE 

BY 

HENRY S. RAWDON, Assistant Physicist 
Bureau of Standards 



ISSUED MARCH 15, 1916 




V/ASHINGTON 
GOVERNMENT PRINTING OFFICE 

1915 






TECHNOLOGIC PAPERS 

1. The Effect of Preliminary Heating Treatment upon the Dr3?ing of Clays (53 pp-)' 

2. The Strength of Reinforced Concrete Beams — Results of Tests of 333 Beams (first 

series) (200 pp.)- 

3. Tests of the Absorptive and Permeable Properties of Portland Cement Mortars 

and Concretes, Together with Tests of Damp-Proofing and Water- Proofing Com- 
pounds and Materials (127 pp.). 

4. The Effect of Added Fatty and Other Oils upon the Carbonization of Mineral 

Lubricating Oils (14 pp.). 

5 . The Effect of High-Pressure Steam on tlie Crushing Streiigth of Portland Cement 

Mortar and Concrete (25 pp.). 

6. The Determination of Chromium and Its Separation from Vanadium in Steels 

(6 pp.). 

7 . The Testing of Clay Refractories, with Special Reference to Their Load-Carrying 

Capacities at Furnace Temperatures (78 pp.). „ 

8- A Rapid Method for the Determination of Vanadium in Steels, Ores, etc., Based 

on Its Quantitative Inclusion by the Phosphomolybdate Precipitate (20 pp.). 
9. Density and Thermal Expansion of Linseed Oil and Turpentine (27 pp.). 

10. Melting Points of Fire Bxicks (17 pp.)- 

11. Comparison of Five Methods Used to Measure Hardness (27 pp.). 

12. Action of the Salts in Alkali Water and Sea Water on Cements (157 pp.). 

13. The Evaporation Test for Mineral Lubricating and Transformer Oils (13 pp.). 

14. Legal Specifications for Illuininating Gas (31 pp.). 

15. Surface Insulation of Pipes as a Means of Preventing Electrolysis (44 pp.). 

16. Manufacture of Lime (130 pp.). 

17. The Fimction of Time in the Vitrification of Clays (26 pp.). 

18. Electrolysis in Concrete (137 pp.). 

19. Physical Testing of Cotton Yams (31 pp.). 

20. Determination of Sulphur in Illuminating Gas (46 pp.). 

21. Dehydration of Clays (23 pp.). 

22. Effect of Overfiring upon the Structure of Clays (23 pp.). 

23. Technical Control of the Colloidal Matter of Clays (118 pp.). 

24. Determination of Phosphorus in Steels Containing Vanadium (11 pp.). 

25. Electrolytic Corrosion of Iron in Soils (69 pp.). 

26. Earth Resistance and Its Relation to Electrolysis of Underground Structures. 

27. Special Studies in Electrolysis Mitigation (55 pp.). 

28. Methods of Making Electrolysis Surveys. 

29. Variation in Results of Sieving with Standard Cement Sieves (16 pp.) 

30. The Viscosity of Porcelain Bodies (g pp.). 

31. Some Leadtek Boro-SIlicate Glazes Maturing at about 1,100° C. (21 pp.), 

32. Special Studies in Electrolysis Mitigation, No. 2. Electrolysis from Electric 

Railway Currents and Its Prevention — Experimental Test on a System of 
Insulated Negative Feeders in St. Louis (34 pp.). 
^^. The Determination of Carbon in Steels and Iron by the Bariiun Carbonate Titra- 
tion Method (12 pp.). 

(Continued on p. 3 of cover) 



DEPARTMENT OF COMMERCE 



Technologic Papers 

OF THE 

Bureau of Standards 

S. W. STRATTON. Director 



No. 59 
standard test specimens of zinc bronze 

(Cu 88, Sn 10, Zn 2 )— PARTS I AND H 
PART I.— PREPARATION OF SPECIFICATIONS 

BY 

C. P. KARR, Associate Physicist 
PART n.— MICROSTRUCTURE 

BY 

HENRY S. RAWDON, Assistant Physicist 

Bureau of Standards 



ISSUED MARCH 15, 1916 




WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1916 



.^ 



^^•;>' 



ADDITIONAL COPIES 

OF THIS rUBLICATIOK MAY BE PEOCTTBED FEOM 

THE SUPERINTENDENT OF DOCtTMENTS 

GOVERNMENT PRINTING OFFICE 

WASHINGTON, D. C. 

AT 

25 CENTS PER COPY 



D. of D. 
APR 12 I9I6 



/6-^^25^/ 



1^ 



STANDARD TEST SPECIMENS OF ZINC BRONZE 
(CU 88, SN 10, ZN 2)— PARTS I AND II 



Part I by C. P. Karr; Part II by Hemy S. Rawdon 



CONTENTS 

PART I 

Page 

I. Introduction and historical 5 

II. Tests recommended and program adopted 8 

III. Methods of preparing test pieces 9 

IV. Methods of molding 11 

1, Testing the molding sand 13 

2. Gating and pouring of the molds 14 

V. Methods of making the alloy 17 

1. Stirring operation 19 

2. Determination of pouring temperatures 20 

VI. Preparation and mechanical tests of specimens 21 

VII. Physical properties 22 

1. General discussion of results of physical tests 22 

(a) Comparison with the work of other investigators 27 

2. Effect of manner of molding 29 

3. Characteristics of typical samples 31 

4. Elastic limit 33 

5. Melting and critical ranges 33 

VIII. Heat-treated specimens 35 

IX. Choice of test specimens 38 

X. Summary and conclusions 39 

Appendixes 42 

Appendix A. — Suggested specifications for the preparation of standard zinc 

bronze (88CuioSn2Zn) test bars 42 

Appendix B. — Comparison with Navy specifications 43 

Appendix C. — Proposed standard specifications of the American Society 

for Testing Materials 44 

3 



4 Contents 

PART n 

Page 

I. Introduction 47 

II. The equilibrium diagram 47 

III. Structure of the cast alloy 50 

IV. Relation between method of casting and microstructure 54 

V. Appearance of specimens after tension test 55 

VI. Correlation of microstructtire and physical properties 56 

1. Type of fracture 6i 

2 . Coarseness of the dendrites 61 

3. Eutectoid 62 

4. Size and orientation of crystals 62 

5. Oxide films and pits 63 

6. Conclusion 64 

VII. Microstructure as related to heat treatment 65 

VIII. Etching of specimens 6§ 

IX. Summary 67 



Part I.— PREPARATION AND SPECIHCATIONS 



I. INTRODUCTION AND HISTORICAL 

The work of preparing an alloy of zinc, tin, and copper for the 
purpose of obtaining a standard form and dimension of test bar 
was undertaken by the Bureau of Standards at the request of a 
committee of the American Institute of Metals. 

Preliminary accoimts of this work have been published in the 
Proceedings of the American Institute of Metals for 191 3 and 
1 9 14, which give a history of the inception of the work and an 
outline of some of the results obtained. 

The zinc bronze — 2 parts zinc, 10 parts tin, and 88 parts copper, 
also commonly known as Admiralty metal. Government bronze, 
and 88-10-2 alloy — has been studied experimentally by a number 
of physicists and metallurgists, but with the exception of the 
investigations of Webbert and of Skillman, all the work done has 
been concerned mainly with the effect of heat treatment at various 
temperatures from 250° to 800° C (482° to 1472° F). These 
previous investigations may be summarized briefly as follows: 

I. M. Bregowsky and L. W. Spring ^ studied the effect of high 
temperatures on the physical properties of some alloys; among 
them were the so-called United States Navy bronze M, and United 
States Navy gun bronze G, which are of the same type as the 
above zinc bronze, but in addition contain small amounts of lead 
and iron. 

Their physical properties, including tensile strength, elongation, 
and reduction of area, were determined at temperatures ranging 
from normal to 520° C (968° F). They found that the tensile 
strength of gun metal G reached its maximimi value, about 37 000 
pounds per square inch, at 300° F (149° C). Its elastic limit 
at normal temperature was 25 000 pounds per square inch, but 
dropped off steadily to about 8000 pounds per square inch at 

1 Proc. Intemat. Assoc, for Testing Materials. Sixth Cangress. New York, First Sec. VII. 

5 



6 Technologic Papers of the Bureau of Standards 

950° F (510° C). Its elongation reached a maximum of 10 per 
cent at about 450° F (232° C), and the reduction of area at 300° F 
(148° C) of 8 per cent, while at 900° F (487° C) both elongation 
and reduction of area become zero. 

In an article on " Metallography as an aid to the brass founder '* 
H. vS. Primrose ^ considered the influence of the pouring tempera- 
ture on the strength of finished castings, the existence of blowholes 
in gun metal, and the examination of test pieces of this material 
under the microscope. Only two pouring temperatures are 
referred to, viz, 950° and 1100° C (1742° and 2012° F), but there 
may be some error or misprint about the first temperature stated, 
because a number of investigators have found the melting point 
of Admiralty gun metal to be about 995° C ^ (1823° F). 

He also shows for two casts of practically the same composition 
that the variation in the temperature of the castings and the 
consequent rate of cooling has a very distinct effect on the structure 
or crystalline formation of the metal. He finds that excessively 
rapid cooling renders gun metal very brittle. 

H. S. and J. S. Primrose ^ read a paper on the heat treatment 
of Admiralty gun metal — 2 parts zinc, 10 parts tin, and 88 parts 
copper — ^wherein they considered the effect of heat treatment 
upon its physical properties at temperatures ranging from normal 
to 800° C (1472° F). Apparently but one pouring temperature 
(a little below 1100° C (2012° F) ) was used in making the alloys. 
They conclude that no improvement may be expected by quench- 
ing, which lowers the strength of the material; annealing of the 
metal for 30 minutes increases Yoxy considerably its strength and 
elongation, the most satisfactor}^ results being obtained after 
annealing 30 minutes at 700° C (1292° F). The homogeneity 
and other physical properties of the metal are correspondingly 
improved, but particularly the ability of the castings to withstand 
hydraulic pressure. These results after heat treatment are con- 
sidered to be due to the removal of the eutectoid from the micro- 
structure, which after annealing shows only the crystals of alpha 
solid solution. 

' Journal of the Institute of Metals, May 24, 1910, Vol. IV, p. 248. 

' H. W. GiUett and A. B. Norton in Transactions of Amer. Inst, of Metals, 1913. Vol. VII, p. 341. (See 
also VII, 2 of present paper.) 
< Journal of Institute of Metals, Vol. EX, 1913, p. 158, 



Standard Zinc-Bronze Test Bars 7 

In a paper on bronzes John Dewrance ^ considered the effect of 
adding small amounts of lead to gun metal. This material was 
submitted to heat treatment at 260° C (500° F), but no data were 
given as to the pouring temperature at which the alloys were made. 

He showed that with 0.5 per cent of lead the breaking stress 
dropped from 16.5 tons per square inch to about 15.9 tons per 
square inch from normal temperature to 550° F (288° C), whereas 
gun metal without lead reached its maximum strength of nearly 
17 tons per square inch at 300° F (148° C), dropped to 16.2 tons 
at 350° F (177° C), and fell rapidly to 9.5 tons at 400° F (214° C), 
and at all higher temperatures up to 700° F (371° C) was inferior 
to zinc bronze containing 0.5 per cent lead. 

In the elongation tests the leaded zinc bronze reaches its maxi- 
mum elongation of 18 per cent at 550° F (288° C) and above 360° F 
(182° C) its elongation is superior to that of the bar containing 
no lead. 

Some additional experiments by H. S. Primrose ^ confirmed the 
fact that only an almost inappreciable change in structure was 
produced by any annealing conducted at a temperature much 
below 700° C (1292° F) , even with 0.5 to i per cent of lead present. 

Messrs. Longbottom and A. Campion ^ studied the behavior of 
Admiralty gun metal at high temperature. They found that 
the maximum stress, about 13 tons, remains practically constant 
to 200° C (392° F), then falls off, reaching almost zero at about 
750° C (1,382° F). The elongation rises to a maximum at about 
150° C (302° F), then falls rapidly, reaching a constant low value 
between 350° to 550° C (662° to 1,022° F). The reduction of 
area follows almost exactly the same course, while the modulus 
of elasticity is constant up to about 200° C (392° F), and then 
falls rapidly. 

L. P. Webbert ^ discussed the strength of nonferrous castings, 
referring particularly to the method of longitudinal gating of 
gun-metal castings, and also considered the direct attachment of 
the test bar to the casting as a means of interpreting the properties 
of the casting. Temperatures were taken at the furnace. 

6 Journal of Institute of Metals, Vol. XI, 1914, pp. 214, 228. 
« Journal of Institute of Metals, Vol. XII, 1914, pp. 254, 256. 

^ Transactions of the Institution of Engineers and Shipbuilders in Scotland. 1914, Newcastle meeting; 
Journal of Institute of Metals, 1914, Vol. XII, p. 281. 

8 Proceedings Amer. Soc. for Testing Materiale, Vol. XIV, 1914, p. 145. 

15271°— 16 2 



8 Technologic Papers of the Bureau of Standards 

The object of these investigations was to note the changes in 
strength that occur in different thicknesses of metal castings 
made from nonferrous metals met with in engineering practice, 
what strength to expect from different alloys, and a study was 
made of different types of test specimens. These tests were 
carried out with the test bar attached to the casting by means of 
a thin gate running from the casting along the longitudinal axis 
of the test bar. 

V. Skillman ^ considered the question of the test specimen of a 
nonferrous alloy in which he referred to the different sand-cast 
shapes and also to a bar cast in an open chill mold, and pointed 
out the desirability of more exact definition of test bars. 

In all of the above investigations some particular phase of the 
problem has been studied, but in none of them have all of the 
variables which influence the ultimate determination of the prin- 
cipal physical properties been taken into consideration, and with 
the exception of Webbert's investigations a sufficient number of 
test bars do not appear to have been examined to give conclusive 
results except in minor particulars. 

Some of the work of these investigators is referred to elsewhere 
in the text. 

II. TESTS RECOMMENDED AND PROGRAM ADOPTED 

The committee of the American Institute of Metals in con- 
sultation with this Bureau considered it would be desirable to 
determine the following properties of this alloy: 

1. Tensile test with determination of tensile strength, elonga- 
tion, and reduction of area, including the recording of the stress 
strain curve. 

2. Compression test. 

3. Determination of (a) heat conductivity, (6) electrical con- 
ductivity, (c) coefficient of thermal expansion, (d) hardness, (e) 
specific gravity. 

4. Shrinkage. 

5. Mechanical properties up to 800° C. 

6. Hydraulic test. \ 

8 Transactions Amer. Inst, of Metals, Vol. VII, 1913, p. 360. 



Standard Zinc-Bronze Test Bars 9 

7. Resistance to corrosion by (a) ammonia, (6) sea water, 
(c) fresh water, {d) Turtle Creek water. 

8. Resistance to erosion by (a) steam, (5) sand blast. 

9. Shear test. 

10. Impact test, 

1 1 . Fatigue test. 

12. Thermal analysis. 

13. Study of microstructure. 

For the present investigation the following tests were finally 
decided upon to be carried out in connection with the methods 
of preparing the alloy : 

1. Experiments to determine the best form of test block and 
test piece. A sufficient quantity of the alloy is to be melted so 
as to cast from the same crucible all of the various blocks pro- 
posed in a paper by Jesse L. Jones ^°, and also a cast-to-size test 
bar both in green and in dry sand. 

2. Tests to determine the best casting temperature, to be made 
after the best form of test block and test piece have been deter- 
mined. 

3. Tests of the following sorts on all the bars cast for the tests 
Nos. I and 2 above: (i) Tensile test, including the determination 
of elongation, reduction of area, and tensile strength, and record- 
ing of the stress-strain curvx; (2) compression test; (3) study of 
microstructure; (4) thermal analysis. 

With the exception of the compression tests, this program has 
been carried out as planned. 

III. METHODS OF PREPARING TEST PIECES 

The operations of casting and molding, etc., as well as the ex- 
periments on the variations in iovm&ry practice, are described in 
considerable detail so that the exact bearing of the results obtained 
may be the more readily appreciated, and for the reason, well 
known to practical foimdrymen, that minute variations in seem- 
ingly insignificant details are often crucial in determining the 
resulting properties of the cast metal. 

In all, 10 series of test blocks were cast. (See Table i and Figs. 
I and 3.) The first series consisted of various sizes of test blocks, 

'^ Transactions Amer. Inst, of Metals, Vol. VI, 1912, p. 173. 



lo Technologic Papers of the Bureau of Standards 

grouped about a central gate and poured flat in green sand. The 
second series was poured flat in dried sand; the first half of this 
series consisted of the same patterns arranged in practically the 
same way as the first series, as shown on the first diagram of Fig. i . 
The latter half of the second series was so arranged that the pat- 
terns were of only two difl'erent kinds, viz, the cast-to-size shape 
and the i-inch diameter cylindrical shape which balanced each 
other and corresponded more closely to each other in mass. (See 

I^ig. 3-) 

The third series was poured vertically in dry sand as indicated 
on the second diagram of Fig. i . 

The barrel and cylindrical shapes of the fourth series were poured 
vertically in dr\^ sand with the bulb reser\^oir at the entrance to 
each mold, as shown in the diagram c of Fig. i . As the cast-to- 
size shape did not prove successful by this method, the potuing 
head was transferred to the end of the nmner and the mold was 
suppHed with metal from one side of the nmner, all other condi- 
tions remaining the same. 

The fifth series was poured flat in green sand, as indicated by 
the diagram d of Fig. i. 

The sixth series was potired flat in dry sand, as indicated by the 
same diagram of Fig, i. 

The seventh series was poured vertically in dry sand, with bulb 
attachment to test block, and with the poiuing head at one end of 
the runner. 

In series VIII all specimens were resented for heat- treatment 
tests. Part of these were potued flat in dry^ sand and the other 
part poured vertically in dry sand. 

The ninth series was distinguished from all others by the fact 
that the metal used consisted entirely of turnings accumulated in 
machining the test bars cast in all of the previous series. Part of 
this series was poiured in an iron chill mold, and part as indicated in 
Fig. I, d. 

The tenth series was poured in two ways, one-half poured flat in 
dry sand and the other half poured vertically in dry sand. Bulb 
attachments were pro\dded at the entrance of each mold, and the 
pouring head was arranged opposite one end of the runner. It 
was also distinguished from the other series by reason of the metal 



Standard Zinc-Bronze Test Bars 



II 



used, v/hich consisted of gates, runners, sprue heads, and floor spill- 
ings from previous casts; in other words, all the metal had been 
used before and in foundry parlance was w^hat is known as "re- 
melted metal." 

TABLE 1 
Schedule of Castings 

[Shapes: A=cast-tosize, B=cylindrical, C=barrel, D=chill.l 



Series 


Shapes 
used 


I 


4 


" 


4 


m 


2 


IV 


4 


V 


4 


VI 


4 


vn 


4 


vin 


3 


IX 


2 


X 


2 



How poured 



Patterns 

in each 

flask 




Metal feed 



Range of 
tempera- 
tures 
covered 



Flat; green sand. 
Flat; dry sand 



Vertical; dry sand. 
do 



Flat; green sand 

Flat; dry sand 

Vertical; green sand • 

Flat and vertical; dry and green sand. 
Flat and vertical; dry sand and chill. . 
Flat and vertical; dry sand 



A, B, C, D 


Plain runner. . 


A, B, C, D 


do 


A,B 


do 


A,B,C,D 


Bulb gate 


A,B,C,D 


do 


A,B,C,D 


do 


A, B, C, D 


do 


A, B, C 


.... do 


A, D 


do 


A, D 


do 



1050-1230 
1060-1240 

1050-1230 
1075-1395 
1190-1325 
1120-1355 
1180-1225 
1120-1345 
1055-1270 
1255-1270 



o First half. 



Second half. 



IV. METHODS OF MOLDING 

The molding sand used for the body of the mold was No. 2 
Albany, mixed with pit Ottawa sand, which is almost pure silica. 
The facing sand was composed of oo Crescent sand, two parts to 
one part bench sand. 

The molding was started by mixing two parts of old molding 
sand to one part of new Albany sand. To 1 1 pounds of this mixed 
sand, 1% pounds of pit Ottawa sand were added and the whole 
thoroughly mixed together. This mixture worked admirably 
and this treatment produced an open-bodied sand well adapted 
to green-sand molding operations. 

For dry-sand molding the method piursued was somewhat 
different. The body of the mold was composed of old, new, and 
Ottawa sand of about the same proportions and thoroughly mixed. 
In putting up the molds the body sand need not be so dry as for 
green-sand molding. When the mold was made it was sprayed 



12 



Technologic Papers of the Bureau of Standards 



with a solution of molasses and water to such a consistency that 
the liquid left a decidedly stick}^ sensation to the fingers. To 
obtain this condition make a mixture of tw^o parts of sirup to 
three parts of water by volume. Each mold was then placed on 
the shelf of a drying oven and as a rule dried for 20 minutes; if 
an inspection of the dried mold showed any indication of moisture, 
evidenced by the darker color of the sand, the mold was again 
dried until this discoloration disappeared. The hot molds were 



' Ser:e3 I Tlat. green Sand 
Series JI OstPccri) FM dry San<^- 



^>-^ 



rirnne 



P7p6 



ni> 



u 

^r ^^!- -^ Hfr 




*' Series JI C^nd Pari) Tlat dry Sand 
•Series JTI Vertical dry Sancl 




c, Series 7V Vertical dri/ Sand 
.Series Wr Vertical gree-r'Sdnd 




Series V. Fiat, green Sand 
Series Vr Flax dry Sand. 



Fic 



■Methods 0/ pouring bronzes. Series I-VII, inclusive. 



allowed to become perfectly cold on the pouring bench before 
they were closed up and poured. The ramming of the flask when 
test bars are to be molded is an important part of the operation. 
The body sand was shoveled into the flask in the usual manner, 
filling it up to about iK inches above the rim of the flask, then it 
was crowded do^vn into place by hand and packed by a kneading 
pressure of the fingers, starting at one edge and working ail arotmd 
the edge of the flask to the point of beginning and then gently 
kneaded down through the center portion. Then the whole sur- 



Standard Zinc-Bronze Test Bars 13 

face was rammed lightly with the wedge-shaped end of the rammer, 
followed by a uniform, light ramming with the rounded end of the 
rammer. The whole surface was then sprinkled over with loose 
sand from the tub and stricken off smooth with an iron bar; sand 
was then slightly spread over the surface in the usual manner, 
smoothed and crowded down with the moldboard. The smoothed 
siudface was tested at different places by finger pressure to deter- 
mine whether the ramming had been uniform, especially if the 
flask was to be poured flat, or if poured vertical the lower part of 
the flask was rammed harder than the upper surface so that the 
casting would not be swelled when poured. 

If the flask was to be potired flat, the pouring hole was molded 
about the wooden plug, and the sand about the plug was rammed 
much harder than the rest of the mold, so as to resist any possible 
cutting action of the pouring stream of metal that might impinge 
upon the sides. 

In*- dry-sand molds the runners and the potuing head were 
dusted with powdered graphite and slicked smooth by a light 
pressure of the fingers before the surface was sprayed with molasses 
water. In order to insure that the molds contained no loosely 
adhering particles of sand which might be dislodged by the 
pouring stream, it was foimd desirable to carefully inspect the 
nmners and pouring heads throughout their entire area and 
length, and where any roughness showed, due to cutting of gates 
or passages, to smooth such surfaces down by a pressure of the 
thumb or fingers, and also to avoid all sharp or angular turns in 
the metal channels. 

1. TESTING THE MOLDING SAND 

According to the best foimdry practice, the quality of a molding 
sand is considered to depend upon its cohesiveness, its refractori- 
ness, textm-e, porosity, permeability, and durability. These 
factors have been clearly defined in a brochure on Foundry Sands, 
prepared by Heinrich Ries and J. A. Rosen.^^ The texture of 
the sand is determined by observing the percentage of the sample 
which is retained on sieves of different mesh. This test is the one 

" Published by the Board of Geological Survey of Michigan, May 6. 1908. 



14 Technologic Papers of the Bureati of Standards 

most commonly referred to as the foundry test for molding sand. 
Messrs. Ries and Rosen advocate the following method: 

Fifty grams of the sand are put into an 8 -ounce bottle and the 
latter half filled with water. The mixtmre is then placed in a 
mechanical shaker for half an hour in order to disintegrate it, 
after which it is washed through a set of 20, 40, 60, 80, and 100 
mesh sieves. The sand on each is dried and weighed. The amoimt 
passed through the lOo-mesh sieve is received in a glass jar. 
WHien all of the water and suspended matter has run through the 
sieves, the contents of the jar are stirred up and allowed to stand 
45 seconds. The amount that settles during this interval of time 
consists almost entirety of fine sand and grains of silt, ranging 
from T3-0 to YTD" inch in size and is classified as ^h^- The water 
with the suspended clay is then decanted off. Since some clay 
is drawn down with the silty particles, the washing process is 
repeated in order to remove the remaining clay. The water over 
the silt and fine sand is removed in part by decantation and the 
residue evaporated to dryness as is also the solution containing 
the suspended clay. If the sample be a bank-molding sand, it 
should first be passed through a lo-mesh sieve to remove twigs, 
other organic matter, and gravel. 

The percentage of material retained on each sieve is found by 
dividing the weight of sand on each sieve by the total weight of 
the sand and multiplying by 100. The majority of molding sands 
have a specific gravity of 2.6. 

The object of the sieve test, as above, is to determine the tex- 
ture of a molding sand. This property is of primary importance. 
It may affect the cohesiveness of the sand; it stands in close 
relation to the permeability of the sand, and to a large degree 
determines the grade of metal that can be cast into it. 

2. GATING AND POURING OF THE MOLDS 

Fig. I shows the method of gating piu-sued for the first seven 
series. (See p. 12.) 

The eighth series was poured vertically in dry sand, somewnat 
like the sixth series, with this exception, that although the rimner 
location was the same the fiask was tilted up on end. The potuing 
head was placed opposite the end of the runner so that the molded 



Standard Zinc-Bronze Test Bars 15 

depressions would be filled up from the side as the molten metal 
rose up from the bottom of the runner. The circular opening 
marked "pouring head" on the figure was closed up and formed a 
pocket at the bottom of the runner, which caught and retained 
nearly all of the oxides, dross, and dirt which had risen to the top 
surface of the pot, but which in pouring was cast into and filled 
the pocket first. This device afforded the molded depressions a 
chance to fill up with clean metal. 

The ninth series was poured in a chill bar and from timiings 
run down directly in the pot. 

The tenth series was poured flat in dry sand in the same manner 
as the sixth series. 

In the first series and first half of the second series the gating 
was found to be defective, inasmuch as the molds to be filled from 
the center runner were of different dimensions, and, since they 
drew from the same supply, imequal shrinkages were produced; 
hence, uniform results could not be seciured. In addition to this, 
the heavier sections showed results inferior to the thinner sections 
because in machining each specimen down to a uniform standard 
section the skin formed at the quickly cooled surfaces of the 
larger specimens was cut away, leaving only the softer core to 
withstand the stresses, which in the case of the thinner specimens 
were imposed upon the more quickly chilled harder shafts. This 
slower cooling of the heavier pieces permitted of a different crystal 
formation and a different phase formation in the solid solution 
of the metal than would be possible in the thinner and more 
rapidly cooled sections, and consequently in the latter case pro- 
duced much higher tensile strength. To obviate these defects 
the patterns were changed in the last half of the second series so 
that they were more equal in volume, more uniform, and better 
results as to tensile strength and ductility were immediately 
secured. 

In the last part of the fourth series the gating was changed, as 
represented by the dotted lines, which represent the new position 
of the pouring head, the old potudng head being stopped off. 

In the seventh and all subsequent series poured vertically, the 
diagram sho^vn for the fifth series, with the dotted lines, indicates 

15271°— 16 3 



1 6 Technologic Papers of the Bureau of Standards 

the position of the pouring head, the old pouring head having 
been closed up and used as a sump to catch slag and scoriae. 

The process of pouring the chill bars furnished some data of 
importance. To make this clear it is necessary to explain that 
the chill bars were potired as flat as possible, almost level, in fact; 
the filHng of the mold began at one end and then the metal flowed 
on to the farther end. In the pouring, the scum and dross which 
were carried down by the pouring stream accumulated in the front 
end of the mold; this sciun, which chilled faster than the molten 
metal, was more sluggish in its flow, and was also held back more 
or less by the frictional resistance of the sides of the mold. When 
machined down it was foimd that as a rule the specimen at this 
end of the mold was softer than at the far end which contained the 
finer metal, and that sometimes the penetration of the dross into 
the mold had reached into the core of the specimen itself. Now, 
the practice had been established, for the purpose of identification 
of the specimen and its pouring position, of numbering the pouring 
end first. It was found as a rule that the far or purer end of the 
mold yielded the more perfect specimen, the one that gave the 
highest tensile result and the greatest elongation. To illustrate 
this point attention may be called to the results on a chill bar, 
cast in the fotirth series, at a temperatirre of 1150° C (2102° F). 
This block is always long enough to make three finished test bars. 
Their numbers were, respectively, 478, 479, and 480. Their 
tensile strengths were 37 800, 42 200, and 45 400 poimds per 
square inch, and their elongations 4 per cent, 8.5 per cent, and 11 
per cent, respectively. Now 4.5 per cent is considered a good 
elongation for a chill bar. It may be seen from these results how 
important it is that the poiuing stream should be free from dross 
and scum of all kinds, and this result may be promoted by the 
use of a poiuing lip, or of a cover over the metal which will hold 
the dross back. It also shows another interesting fact — that with 
pure metal, the higher the tensile strength of a specimen the 
greater is the elongation that may be expected. The standard of 
performance in such a case may undoubtedly be raised by the use 
of the greatest of care to insiire the piuifying of the alloy. 

In the third series (Fig. i , h) the patterns were better balanced 
and arranged on opposite sides of the ninner, and although internal 



Standard Zinc-Bronze Test Bars 1 7 

stresses still developed through inadequate feeding supply fairly 
good results were secured. 

In the fourth series (Fig. i , c) a bulb reservoir was for the first 
time molded with the test piece opposite each mold, which sup- 
plied an adequate amount of metal to take up the shrinkage. By 
this device internal stresses developed by cooling were almost 
wholly avoided. In this series the center mold proved defective 
because it was directly opposite the pouring gate. This arrange- 
ment permitted the center mold to catch all the dross, oxides, and 
scoriae natural^ carried down by the entering column of molten 
metal. To avoid this the poiuing head was shifted to one end of 
the runner without changing the position of the patterns in the 
molding operations, whereupon better results were immediately 
obtained. This bulb, as indicated on Fig. i, was about 2% inches 
in diameter. 

The fifth, sixth, and tenth series were gated as shown in Fig. i , d. 
The seventh was gated in the same manner as the revised fotulh 
series; that is, the pouring head was placed opposite one end of the 
runner, and the circular molded portion acted as a pocket or sump 
to collect the dross and scoriae so that the molds could be cleanly 
fed with metal. Unless sufficient space is provided in the runner 
and in the pouring head for the metal to enter the mold in a solid 
steady stream, and at the same time to allow sufficient space adja- 
cent to the descending column of metal, the gases developed will 
have no opportimity to escape and free themselves from the metal. 
The consequences will sometimes be a choked mold, which does 
not fill up uniformly, or castings that have become permeated 
with gas holes, and therefore porous, or both undesirable results 
may accrue. To secure good results, it therefore becomes neces- 
sary to have the nmner molded with the patterns so as to secure a 
smooth and uniform passage for the flow of metal, a channel which 
can not be obtained by cutting out after the mold is made. 

V. METHODS OF MAKING THE ALLOY 

The fmnace used was a cylindrical, brick-lined chamber with 
separate removable iron grate bars, and provided with a conical- 
shaped cast-iron cover. It was of the pit t)rpe, coke fired, with 



1 8 Technologic Papers of the Bureau of Standards 

natural draft, and was provided with a chimney about 60 feet in 
height. It was found necessary to break the coke to a uniform 
size, as the space between the pot (a No. 40) and the furnace Hning 
was only about 2% inches in width. In the bottom of the furnace, 
resting on the grate bars, an old pot was cut dov^-n to about two- 
thirds of its former height and was inverted to form a permanent 
support for the melting pot, so that the latter would not settle 
down on the grate bars as the coke burned out, and also for the 
purpose of keeping the melting zone at a fixed height, \dz, about 
4 inches above the bottom of the pot. The average time of melting 
the charge of about 65 poimds was about two and one-fourth hours. 
The fuel used was 7 2 -hour coke, kept 6ry. One heat per day was 
poiured. 

The pot was charged as follows : In the bottom an inch layer of 
charcoal broken into pieces about the size of a hickory nut was 
placed evenly. The copper ingots of the weighed charge were set 
on this bed of charcoal, small pieces of gates, sprue heads, or 
defective castings were then put in to fill the crevices between the 
ingots of copper and to level off the surface up to the top of the pot. 
Broken charcoal, very small pieces with no dust, were added as a 
protective covering, and the graphite cover was placed over the 
entire charge. The coke fire was then urged to its utmost. As fast 
as the body of coke burnt down what remained was crowded 
down firmly with a heaw iron bar, fresh coke added up to the top 
of the pot and sometimes strewn over the cover itself, and the firing 
then proceeded until the charge was melted. The graphite cover 
was then removed and the molten metal stirred. When the copper 
was completely melted, the tin was added and carefully stirred in. 
When the metal had come to a somewhat higher temperature, the 
zinc, after being pre^dously heated, was added and stirred into the 
molten bath as rapidly as possible. All of the charge now being 
in the pot and ascertained to be in a molten state, the fire was 
accelerated and the charge heated up considerably higher ; in fact, 
until lambent zinc flames began to be seen around the edges of the 
metal stuface adjoining the pot. 



Standard Zinc-Bronze Test Bars 19 

1. STIRRmG OPERATION 

The object of stirring the molten metal thoroughly was to pro- 
mote the intimate mixtm-e of the components of the alloy. If 
the stirring rod be worked up and down and from side to side, as 
well as moving about in a circular path, the various particles of 
the alloy will be brought into more intimate contact than if only 
one kind of motion be followed in the stirring. The rotary motion 
is the one generally used, but if also accompanied by a vertical 
pumping movement of the stirring rod, the heavier particles at 
the bottom will be displaced, the lighter particles dragged down- 
ward, and a thorough diffusion of all the particles and greater 
uniformity of mixtmre will be obtained. 

A graphite rod is inserted into an iron sleeve about 12 inches 
long, into which has been cut a Vio-iiich slot. This sleeve is set 
in an iron collar 1^732 inches inside dimension by ^/i inch long, 
which is secured to the sleeve and tightens the clutch on the 
graphite rod by means of a %-mc\i standard set screw. At one 
end of the sleeve the rod is inserted and the other end is screwed 
into a ^-inch iron pipe, which is made of any length desired. 
(Fig. 2.) 

There is the objection to both the iron and graphite rod that 
they absorb heat from the molten raetal and reduce its tempera- 
ture if the rods be introduced in a cold state; they must be 
heated to a cherry red heat before being plunged into the metal. 
The disadvantage of the iron rod is that if it be kept in the metal 
long enough to stir the mass effectively little globules of the iron 
drop from the iron poker into the metal alloying with and con- 
taminating the melt. 

The advantages of the graphite over the iron rod for stirring 
purposes may be readily seen by considering the fact that the 
consumption of the graphite rod in contact with the molten 
metal produces a gas and does not furnish any element capable 
of combining with the constituents of the alloy unless it be the 
silicon which may be present in only such a slight amount that 
no deleterious effects are perceptible in the cast alloy; this result 
is confirmed by the fact that chemical analysis fails to show any 
silicon in the alloy. The rod burns awa}^ gradually, the combus- 



20 Technologic Papers of the Bureau of Stayidards 

tion of the rod taking place chiefly at the bottom until the i>^ 
inches diameter of the section bmns away to a point. The rod 
may be used safely, as a rule, for not less than 30 consecutive 
heats, and this average may sometimes be increased to 40 heats. 
A very satisfactory rod is one made of Acheson graphite, which 
is almost pure carbon. It is a soft, dense, brittle substance, and 
in its use as a stirring rod must be handled with some care. A 
rod of so large diameter as specified seems capable of offering the 
transverse resistance to fracture required by the stirring opera- 
tion. Its fragility comes into question if it be carelessly dropped 
from the workman's hand when the rod has serv^ed its purpose. 
A good method of preser\ang it from ti'ansverse fracture is to 

3;' Standard Set Screjr 
^ushiTjg f • Eytra 2^vu Q Jron CoUar 1 3k I-D 5y276 02f Sy a '»* 

j ^ ^ ^^^--^ 

, !__-. y / tarn] 



1, *< 

hf^a-ndard Wm'i Iron 7>ipe 



- '/6 Satrs'3 Sfoi 

.Jl 



— >. /2.' ^tandar<f H^o'f Iron 'P(jp«. 



/2- -^ 

Fig. 2 

plimge it vertically into a barrel of loosely packed sand immedi- 
ately after being withdrawn from the furnace. 

' 2. DETERMINATION OF POURING TEMPERATDRES 

The temperature was taken while the metal was still in the 
furnace by means of a Holbom-Kurlbamn type of the Morse 
optical pyrometer, or after the pot was removed from the fire, by 
means of a thermocouple consisting of platinum, platinum -iridium 
(lo per cent). The platinum couple was incased in an inner tube 
of quartz open at both ends for the insulation of the two wires 
from one another, and the whole sheathed in- a quartz tube closed 
at one end. The closed end of the quartz tube was held over the 
molten metal until the temperature of approximately 600° C 
(1112° F) was indicated, and then plunged to within about 6 
inches of the bottom of the pot. The needle of the registering 
millivoltmeter rose steadily and rapidly to a constant position 
when the reading was taken, and there was no perceptible lag 



Standard Zinc-Bronze Test Bars 21 

noticed after the couple had been plimged into the metal. It was 
foimd that all slag or scoriae adhering to the tube from the con- 
tents of the pot could, when cold, be removed easily by one's 
fingers without detriment to the tube. A set of tubes was used 
for months without any marked deterioration. 

The thermometer used for taking the temperature of the cold 
junction was wound about with one of the copper leads from the 
Pt wire of the thermocouple at one end and the other end wound 
about in the same manner with copper lead of the Pt-Ir wire. 
The thermometer was fastened to a thin board with insulated 
wires and the whole secured to the upper end of the outer quartz 
tube so as to become rigidly a part of the device. This method 
prevented any tmdue pulHng strain on the tubes or the connec- 
tions, and this arrangement became a practicable foundry device, 
with which the poming temperature of one or more pots could be 
taken successively. It was found that after a month's training 
and experience almost any intelligent workman could take the 
pouring temperatures acctirately and rapidly. This experiment 
was undertaken to show that although operating under labora- 
tory conditions the thermocouple as constructed would be appli- 
cable to general foimdry conditions and requirements. 

VI. PREPARATION AND MECHANICAL TESTS OF 
SPECIMENS 

All of the various shapes cast in sand were machined down to 
the uniform length of 4K inches, with a center test section 0.505 
inch diameter by 2 inches long, and fillets connecting the threaded 
ends }i inch diameter with the center section. Three points, i 
inch apart on centers, were estabUshed on the center section of 
each bar, from which to measure the elongation after stress. 

Since the ninth series was poured in chill bars 13 inches long, 
it was possible to secure three test bars from each chill bar. These 
were turned down to a tmiform diameter of about i inch, and cut 
into three pieces which were further machined down to the re- 
quired dimensions. 

Each bar was tested in an Emery hydraulic testing machine. 
Extensions were taken by an extensometer for increasing loads 
of 200, 500, 1000, 1500, 2000, 2500, 3000, 3500, and 4000 pounds. 



22 Technologic Papers of the Bureau of Statidards 

After the 4000-pounds load the extensometer was removed from 
the bar and the stress continued up to complete destruction of 
the bar, the breaking load being recorded and the stress per square 
inch calculated from the dimensions of the section. Then the 
elongation on each part was measured and the sum of the two 
taken as the total elongation in 2 inches. The reduction in area 
at the point of fracture was taken by means of a micrometer. 

VII. PHYSICAL PROPERTIES 

All of the above-mentioned data were obtained for each speci- 
men and the curves plotted from the information thus obtained, 
as shown in the Figs. 5, 6, 7, 8, and 9. While all of these physical 
properties have been plotted in consecutive sequence with refer- 
ence to the temperatures at which the bars were poured, with the 
object of determining the effect of pouring temperature on the 
properties of the alloy, there appears to be, nevertheless, no sim- 
ple relation readily deducible between the pouring temperature 
and the strength of the specimen. 

1. GENERAL DISCUSSION OF RESULTS OF PHYSICAL TESTS. 

The physical properties to be considered were the tensile 
strength, elongation, reduction of area, extensions under increas- 
itig loads from 200 to 4000 potmds, respectively, and the hard- 
ness and specific gravity. The test bars were cast as separate 
units and not as attached members to the body of a larger casting, 
for it is now admitted that for a copper tin alloy the attached test 
bar does not represent the strength of the casting as a whole and, 
in general, would be expected to give higher values than sections 
cut from the castings themselves. To show the relative values the 
metal is capable of giving, the various shapes were cast as sepa- 
rate units, with different methods of molding, and poured at 
various temperatures to ascertain, if possible, the best methods 
to be ptirsued in order to obtain uniform resiilts. It has also been 
found by various experiments that when test bars were made of 
imequal sectional area, and the test members turned from the 
larger pieces, the interior possessed no skin and gave lower re- 
sults. (See Webbert on "Specimens for non ferrous castings.")" 

^2 Proc. of Amer. Soc. fcr Testing Materials. 



Standard Zinc-Bronze Test Bars 23 

It has also been found that owing to a lack of uniformity in the 
rate of cooling of the heavier bars (exceeding ^ inch diameter) 
the physical results show considerable lack of uniformity. For 
these reasons the greatest number of experiments in this investi- 
gation have been made with specimens that have not had the 
external skin cut away very much in reducing them to the size 
required for testing. Another advantage of the smaller size and 
almost uniform section is th^ the shrinkage is more uniform and 
when such sections are properly gated there is less tendency to 
the production of internal strains, which would develop a weak- 
ness in the tested specimen, and thus lead to erroneous deductions. 

It can be shown that the tensile strength alone of any given 
specimen is not an invariable criterion of its value, and the same 
deduction is true of the elongation and reduction of area; also that 
there is a closer relation and interdependence between the elonga- 
tion and the reduction of area of a specimen than between any 
two other properties. A large elongation in a sand-cast specimen 
is generally accompanied by a corresponding increase in the reduc- 
tion of area. In the tests made of the extensions under successive 
increasing loads the different bars showed no definite yield point 
except in a few limited cases, but wherever the stress-strain curve 
plotted was uniform in curvature the bar would show a uniformity 
of texture from which superior results might be expected. Wher- 
ever there is a sudden deviation from this imiformity of deforma- 
tion it is probably due to some interior stress developed at that 
point by the load of deformation, as such specimens as a whole 
fail to reach the normal average of the bars of such a series. The 
conclusion to be derived from these results is that imiformity of 
curvature of the stress-strain ciurve is a safer guide to the homo- 
geneity of the mass-structure than the so-called yield-point deter- 
mination. It is known that once the yield point is attained further 
loading increases the elongation of the bar with more than normal 
rapidity. 

It is well known from metallographic studies that during the 
cooling of an alloy from the fluid state, crystals separate out from 
the mother fluid, which possess a density different from that of 
the body of the fluid, and these heavier portions sink to the bot- 
tom. Such a phenomenon is known as liquation. This pheno- 

15271°— 11 4 



24 Technologic Papers of the Bureau of Standards 

menon is favored by a slow rate of cooling, hence the larger the 
sectional area of the test bar the less will be the probable homo- 
genity, and this fact is an additional reason for choosing the 
smaller section in the making of the test bar. The tendency to 
liquation is avoided by a thorough stirring of the molten bath 
before the bar is poured, followed by a rapid cooling, where such 
a procedure is possible. In test bars that show a uniform texture 
at the fracture but yield low tensile results, a metallographic 
examination of the fresh fracture will sometimes reveal tiny 
globules, or bright surfaces, which appear to fill up the interstices 
between the crystals; this appearance shows that some alloy 
phase of a melting point lower than the mother fluid has solidified 
last. This formation is always attended by results lower than 
normal, and is due first to inadequate stirring of the metal when 
compounded, or to too great a proportion of virgin metal, or to 
ineffective stirring just before the molten alloy is poured into the 
molds. It is not of frequent occurrence except when lead is a 
component of the prepared alloy. 

From the chill bar, which presents the minimum elongation to 
be expected, with a given tensile strength, to the cast-to-size 
shape, cylindrical form, and so on, the larger the sectional area 
at the center of the piece for the same length of the specimen, 
the greater is the elongation that may be expected, but at the 
same time there is no factor from which, if one of the values be 
known, the other may be derived. 

The chill-bar test is not representative of the alloy imless the 
bar be annealed or quenched. Poiured from the same heat as 
the sand-cast specimen, its rate of cooling is more rapid, its den- 
sity is greater, and its crystalline structtue is not so open in texture 
as the sand-cast specimen, and for these reasons it is not compar- 
able in the physical properties developed under stress to the sand- 
cast specimens. 

In considering the physical properties of the chill-bar specimen 
it is well to remember that at rather low and very high pouring 
temperatures the chill bar represents the utmost tensile strength 
of which the metal is capable, and only on account of its sudden 
solidification is its corresponding percentage of elongation and 
reduction of area greatly lowered over what is expected of the sand- 



Standard Zinc-Bronze Test Bars 



25 



cast specimen. This initial chill creates internal stresses and 
prevents the flow of the metal imder continuous tensile stresses. 
In other words, the internal core of the bar is in an abnormal 
state; now if this state be normalized by restoring its internal 
structure to a condition of equilibrium at the surface with respect 
to stresses, by submitting such a chill bar to a proper heat treat- 
ment, either annealing or quenching after heating to a temperature 



--N- 






-*, 



-(•k- 




f^ 



^7/6^ 






"'m 



Fig. 3. — Patterns of test bars 



of from 700° to 800° C (1292° to 1472° F) for 30 minutes, it will be 
found either in the properties of elongation or reduction of area 
or in the metallographic examination, that the micros tructure 
has been normalized, and that the elongation and reduction of 
area have been increased by nearly 400 per cent, giving values 
equivalent to those of the best sand-cast specimen that can be 
obtained. In investigating a chill-bar specimen it would be advis- 



26 



Technologic Papers of the Bureau of Standards 



able, therefore, to submit it to a proper heat treatment in order 
to restore the metal to its normal state and then to test it for its 
true physical properties, because the heat treatment recommended 
does not deform the specimen in any way nor does it alter its 
chemical composition. 

The general results show that there is no proportionate ratio 
between elastic limit and tensile strength, but a more extended 
investigation of these properties of suitably heat-treated, zinc- tin 
bronzes may be able to establish such a relation 

Fig. 4 is a photograph of 14 specimens which shows their ap- 
pearance after fracture; photomicrographs of fractures and sec- 
tions of typical specimens are also shown in Part II. 

Table 2 furnishes details of the shapes, pouring temperatures, 
method of molding, and physical properties. 

TABLE 2 



Test bar 



Shape 


Pouring 
tempera- 
ture 


Sand 


Tensile strength 


Pounds 
per sq. in. 


Kilograms 
per cm2 


1 

i ^ 


1200 


F. G. S. 


45 700 


3213 


B 


1200 


F. G. S. 


40 500 


2847 


c 


1220 


F. D. S. 


37 800 


2657 


A 


1200 


F. D. S. 


45 000 


3164 


B 


1120 


F. D. S. 


39 300 


2763 1 


B 


1220 


V. G. S. 


41 800 


2939 


D 


1220 


ChiU 


49 700 


3494 


A 


1225 


V. G. S. 


49 600 


3487 


A 


1180 


V. G. S. 


49 100 


3452 


A 


1255 


F. D. S. 


50 500 


3550- 


D 


1120 


Chill 


49 300 


3466 


A 


1255 


F. D. S. 


35 600 


2503 


A 


1270 


V. D. S. 


29 300 


2060 


A 


1270 


V. D. S. 


42 500 


2988 



Elonga- 
tion in 2 
inches 



Reduc- 
tion of 
area 



531.. 
591., 
609.. 
637.. 
648.. 
704.. 
707.. 
740.. 
802. 
814. 
905.. 
923.. 
1015 
1020 



Per cent 
24.5 
24.5 
25.0 
21.5 
17.5 
29.0 
6.0 
30.5 
36.0 
53.0 
' 5.5 
16.0 
9.5 
18.5 



Per cent 
20.0 
23.0 
23.0 
18.0 
17.0 
24.0 

7.0 
19.1 
33.7 
40.3 

4.0 
15.0 
16.9 
20.2 



[Shapes: A=cast to size, B= cylindrical, C=barrel, D=chill. Sand: F. D. S.= poured flat in dry sand. 
F. G. S.=poured flat in green sand, V. D. S.=poured vertical in dry sand, V. G. S.= poured vertical in 
green sand, Chill= poured in open iron chill moldj 

By referring to the table it will be noted that in specimens 531 
and 591 the elongation was identical, but there was a difference 
in their reductions of area and tensile strength in which the cast- 
to-size shape exceeded the cylindrical shape by over 5000 poimds 



Bureau of Standards Technologic Paper No. 59 





Fig. 4. — Fractured test bars 



Standard Zinc-Bronze Test Bars 27 

per square inch. Although not poured at the same time from the 
same heat, the comparison is a fair one, because both were poured 
at the same temperature. It is probable that where every other 
condition is the same the cylindrical shape will yield a specimen 
from which might be expected a higher relative elongation in 
proportion to its tensile strength than could be obtained from the 
cast-to-size shape. 

In the three shapes representing the sixth series the barrel- 
shape specimen shows its superiority in elongation and reduction 
of area to either of the other shapes, but, as might be expected, 
was inferior in tensile strength. 

In the sand-cast specimens representing the seventh series the 
same relative high percentage of elongation in proportion to ten- 
sile strength is shown by the cylindrial specimen. 

The eighth series consists entirely of specimens which have been 
submitted to heat treatment. (See Sec. VIII.) 

The two specimens from the ninth series stand at the beginning 
and the completion of the zone of favorable pouring temperatures, 
and the results obtained may be considered as representative. 
Both were poured from ingot metal made from turnings of previ 
ously prepared specimens and contained no virgin metal. 

The two specimens from the tenth series were poured at the 
same temperature but in different heats. Both show great uni- 
formity of extension from the initial to the ultimate load. The 
great difference in the values obtained is probably explainable by 
the inclusions of oxide segregations or the presence of inclosing 
oxide films about adjacent crystals, which would lessen their 
tensile strength. 

The chill bars 707 and 905, respectively, exhibit the character- 
istics of all the test bars poured in this series of tests, viz, a low 
elongation and reduction of area corresponding to a high tensile 
strength. 

(A) Comparison With the Work of Other Investigators. — In some 
tensile tests of this same type of zinc bronze recently reported to 
the American Society for Testing Materials Mr. Webbert found 
for eight tests an average tensile strength of 38 866 pounds per 
square inch and 25.3 per cent elongation; the pouring temperature 



28 Technologic Papers of the Bureau of Sta-iidards 

was found to be 1120° C (2048° F), but bis alloy contained i 
ounce of 15 per cent phosphor copper for ever}^ 100 pounds of 
alloy. W^le it is not known exactly how much such an addition 
of phosphor copper will increase the tensile strength of such an 
alloy, it is well known that other conditions being the same it 
would produce a marked increase of the tensile strength and of the 
elongation. 

The same alloy made by H. S. and J. S. G. Primrose, cast in dry 
sand, had a tensile strength of 38 528 pounds per square inch and 
an elongation of 24 per cent, whereas the 18 specimens cast flat at 
the Bureau in dry sand at about the same temperature, 1175° C, 
had an average tensile strength of 40 148 pounds with an elonga- 
tion of 20.3 per cent and a reduction of area of 18,3 per cent. 
Neither in Primrose's nor in the latter specimens was there any 
phosphor copper used. It should also be noted that in the latter 
specimens some 20 to 25 per cent of remelted gates, sprue heads, 
and clean floor spillings were used, which would have a tendency 
to reduce the percentage of elongation and reduction of area 
obtained without materially lowering the tensile strength. 

Primrose's specimens were cylindrical in shape, 10 inches long 
by I inch diameter; some portion of the chilled surface was cut 
away in reducing the specimen to the size required for testing, and 
the fact that they were 10 inches long as against our 4K inches 
might increase his percentage of elongation. By affording the strain 
an opportunity to be more evenly distributed would also tend to 
increase the elongation. 

As an illustration of what a similar bar may do, one of the same 
shape as his poured at a temperature of 1200° C (2192° F) flat in 
green sand showed a tensile strength of 40 500 pounds per square 
inch, an elongation of 24.5 per cent, and a reduction of area of 23 
per cent. An average of these 10 tests shows that it is possible to 
obtain a tensile strength of about 39 000 poimds per square inch, 
with a corresponding elongation of 25 per cent, if only virgin ingot 
metal is used in making the alloy. 

The bar represented by specimen 1015 shows a result below the 
normal. This was the first bar cast in the third flask of the heat, 
the bars cast in the same flask giving an average tensile strength of 



Standard Zinc-Bronze Test Bars 29 

34 600 pounds per square inch, an average elongation of 12.3 per 
cent, and an average reduction of area of 20.5 per cent. This heat, 
as its series number indicates, was poured entirely from old gates, 
sprue heads, and floor spillings, and contained no new metal. It 
was, therefore, poured piurely as a "remelt" test, and, as the tem- 
perature of pouring was 1270° C (2318° F), rather high for a 
" remelt " series of tests, the low result in elongation and reduction 
of area is readily accounted for. 

The bar represented by specimen 1020 was cast in the fourth 
flask of the same series, and poured at the same temperature as 
No. 1015, but was the last one of the series to be poured. The 
average result of the three bars in that -flask was 38 900 pounds 
per square inch, 17 per cent elongation, and 18.6 per cent reduction 
of area. As the temperature of pouring when the fourth flask was 
reached was considerably lower than that at which the third flask 
was pom-ed, it is evident that the elongation and reduction of area 
were influenced by the temperattue at which the specimens were 
poured. To carry the comparison still fiulher, the specimens 
poured vertically in dry sand from the same pot of metal, and at 
virtually the same pouring temperature, show in all the physical 
properties superior results to the specimens poiu-ed flat in dry sand. 

2. EFFECT OF MANNER OF MOLDING 

Fig. 5 consists of two diagrams, one showing the results of the 
tensile tests of the cast-to-size shape arranged according to the 
various methods of molding previously described. In the casting 
temperature zone favorable to all methods the bars cast vertically 
in dry sand show the greatest uniformity of results, because no 
doubt variables of sand conditions were less than in other methods. 
This is due doubtless also to the more rapid filling up of the mold in 
pouring and to lack of cold shuts in the casting; also to the 
greater freedom with which the gases evolved in pouring freed 
themselves from the falling column of molten metal. The second 
diagram shows the results of the tensile tests for each shape but 
with all methods of molding combined. Through the favorable 
range of casting temperatures for all specimens the cast-to-size 
shape is unmistakably superior to all other shapes in its high range 



30 



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issii 



Standard Zinc-Bronze Test Bars 31 

and comparative uniformity of results. The results for the cylin- 
drical and barrel shapes were also very uniform and ran very 
closely parallel. As shown in this diagram, the chill bar, on the 
whole, appears to be superior in strength to the two latter shapes 
as cast in sand. 

Fig. 6 consists of two diagrams showing the results of tensile 
tests of the cast-to-size shape and the chill bar. The chill bar has 
a tendency to both high tensile strength and ductility at both the 
lower and higher range of casting temperatures and shows some 
uniformity of results in the favorable casting zone of temperatures 
1120° to 1270° C (2048° to 2318° F). 

Fig. 7 consists of three diagrams, the upper one showing the 
elongation in 2 inches of all shapes, the middle one the reduction 
of area of the same, and the lower one the range of the tensile 
strengths of the cylindrical and barrel shapes at all temperatiu-es 
potired, together with the number of tests made at each tempera- 
tture. It is noticeable, as might be expected, that the elongations 
of the chill bars at all temperatures except the highest, omitting 
the last one, fall much below those of the sand-cast specimens, 
although maintaining a uniformity, which shows by comparison 
that sand casting itself produces irregularities which are exceed- 
ingly difficult to account for. From the tests of reduction of area 
similar conclusions may be drawn. 

In the comparison of the range of tests — that is, from low to 
high tensile strength — between the cylindrical and barrel shaped 
types the former shows some superiority, although slight, especially 
as to uniformity, but both agree in falling below the normal average 
at the high temperatures. This result is due no doubt to the 
greater oxidation and loss of material by vaporization than when 
potired at the lower temperatures. This is confirmed by the 
results of the microscopic studies. (See Part II.) 

3. CHARACTERISTICS OF TYPICAL SAMPLES 

Table 3 contains the results of the chemical analysis, density, 
and other physical properties of further typical samples. As a 
rule, the larger the percentage of tin contained the denser the alloy, 
and likewise the tensil strength appears to be greater with the 
denser specimens. 
15271°— 16 5 



32 Technologic Papers of the Bureau of Standards 

TABLE 3 
Characteristics of Typical Samples 

Shapes: A=cast to size, B= cylindrical, C=barrel, D=chill cast. Sand: F. D. S.=poured flat in dry 
sand, F. G. S.=poured flat in green sand, V. D. S.=poitred vertically in dry sand, V. G. S.=poured 
vertically in green sand, chill=poured in chill mold. Density=grams per cubic centimeter.) 



Test 
bar 


Shape 


Sand 


Pour- 
ing 
tem- 
pera- 
ture 


Chemical analysis 


Tensile 
strength 


Elon- 
gation 

in 2 
inches 


Reduc- 
tion of 
area 




Cu 


Sn 


Zn 


Poxmds 

per 
sq. in. 


Kilo- 
grams 
per cm2 


Density 


19 


A 
A 
A 
A 
C 
A 
A 

A 

A 

D 


F. G. S. 
F. D. S. 
V. D. S. 
V. D. S. 
F. G. S. 
F.D.S. 
V. G. S. 

r V. G. s. 

tv.D.S. 

V. D. S. 

Chill 


"C 

1155 
1060 
1050 
1175 
1190 
1220 
1220 

} 1120 

1255 
1255 


88.0 
88.7 
87,8 
86.96 
87.15 
87.03 
87.03 

86.67 

86.85 
87.29 


10.2 
9.3 

10.35 

10.91 

10.0 

10.42 

10.40 

10.63 

9.97 
9.65 


1.8 

2.0 

1.95 

2.13 

2.85 

2.55 

2.57 

2.70 

3.18 
3.06 


48 050 

43 550 
42 270 
41 900 
39 700 
39 900 
46 100 

44 000 

39 800 
34 000 


3378 
3061 
2972 
2946 
2791 
2805 
3241 

3093 

2798 
2390 


Perot 

26.0 
30.0 
20.5 
14.0 
31.0 
17.0 
29.0 

20.5 

15.5 
15.0 


Perot 

20.0 
30.5 
13.5 
16.0 
24.5 
16.2 
22.6 

14.0 

13.0 
12.1 




213 




394 

430 

561 

605 

705 

810 

929 

1011 


8.519 
8.590 
8.378 
8.576 
8.777 

8.730 

8.471 
8.618 



No appreciable quantities of chemical elements other than those 
indicated were detected. 

One representative sample has been chosen from each series, 
bearing in mind the necessity of representing the poiuing tempera- 
tures from the lowest to almost the highest in what has been found 
to be the zone of temperatures favorable to the securing of repre- 
sentative types. It should also be remembered that the ninth 
series is poured entirely from turnings. The tenth series is poured 
from old gates, sprue heads, and floor spillings and is purely a 
**remelted" metal. 

The percentages of copper, tin, and zinc, which are foimd in the 
analyzed metal, give an indication of the imiformity and exactness 
with which this alloy can be expected to be rdade up from new 
metal containing no scrap under the various molding and casting 
conditions. The samples for density were taken from portions of 
the sample not subjected to stress. It is seen that the density 
ranges from 8.378 to 8.777 and the slight variations appear to be 
associated with the tin content. 



Standard Zinc-Bronze Test Bars 33 

4. ELASTIC LIMIT 

An examination of Fig. 6 shows that there is no sharp break in 
the stress-strain curve, and that the deformation of the bar takes 
place so gradually and as a rule so uniformly that the yield point 
is difficult to locate, but from the deformations indicated it may 
be shown that this limit ranges from 1 5 000 to 1 7 000 pounds per 
square inch. The specifications of the Bureau of Steam Engi- 
neering, United States Navy, for a bronze of this description 
require a minimum yield point of 15 000 pounds per square inch. 
As an illustration of what a good specimen can show, attention is 
called to test bar 357, which showed a definite yield point under a 
load of 4000 pounds, equivalent to a stress of 20 000 poimds per 
square inch. Many specimens having remarkably good physical 
properties show a definite high yield point, but in general this 
point can not be defined with any great degree of accuracy. 

5. MELTING AND CRITICAL RANGES 

The thermal curves are by the inverse rate method of Osmond 
in which the times required for the specimen to rise or fall equal 
temperature intervals are noted in terms of the temperatm-e or 
dt/d^ vs. B. But instead of using equal temperature intervals, 
equal emf intervals were used, corresponding to approximately 
2° C. The specimens, turned down to about 10 mm diameter 
and 25 mm length, were tested in vacuo in a specially designed 
and automatically regulated electric resistance fumace.^^ The 
emf intervals were measured with a platinum, platinum-rhodium 
thermocouple and a potentiometer, and the time intervals were 
measured to o.i second on a cylindrical, motor-driven Geneva 
chronograph. 

Table 4 gives the results to the nearest 1° C and are probably 
accurate to better than 5° C. It is seen that the melting point, 
about 980° C (1796° F), is preceded by a transformation of con- 
siderable magnitude at 780° C (1436° F), and by a minute one at 
530° C (986° F), which last is apparently suppressed sometimes 
after the first heating. The first corresponds to the final freezing 

" Bureau of Standards Scientific Paper No. 213, Burgess and Crowe. 



34 



Technologic Papers of the Bureau of Standards 



and separation of the beta constituent, the second corresponds to 
the transformation of beta into alpha + delta. There appears 
to be no evidence of a third transformation in this alloy. 

TABLE 4 
Location of Maximum of Critical Ranges in Degrees Centigrade 



Sample 


Run 


Heating 


Cooling 


424 


1st 






845 
847 

843 
a 847 

792 
C790 

792 
C790 

844 

844 

836 
& 808-832 

898 












424 


2d 
















424 


1st.... 
2d. 


528 










787 
h 799-785 
789 
789 
787 
787 




424... . 










490 


1st.... 
2d 

1st.... 

2d 

1st.... 

2d 

1st.... 

2d 

1st. .. 


530 
526 
530 

d529 
526 

(-) 
518 
516 


/781 
781 

781 










490 










9. 










9.. 










19.. 










19 












422 


?978 
982 


ft 981 

978 

h99Z 


817 


776 
775 
785 


504 


422 


504 


380 














Not so well 
6 Double cusE 
c Smaller thai 
^ Very small. 


defined a 
1 on first 


is on firs 
run. 


trun. 




« Disappears after first run. 
/ Small on first run up. 
9 MeltiQg point. 
A Freezing point. 





The physical properties of the specimens submitted to thermal 
analysis are as follows: 

No. 424 was a cast-to-size shape bar, poured at a temperature 
of 1175° C (2147° F). It had a tensile strength of 45 000 pounds 
per square inch, a yield point, determined by the drop of the 
beam, of 19 600 pounds per square inch, an elongation in 2 inches 
of 19 per cent, and a reduction of area of 13.5 per cent. 

No. 490 was a cylindrical-shaped bar, poured at a temperature 
of 1390° C (2534° F). It had a tensile strength of 14 000 pounds 
per square inch, a yield point (by drop of beam) of 4500 pounds 
per square inch, an elongation in 2 inches of ' i per cent, and a 
reduction of area of 0.5 per cent. Examination of the structure 
exhibited a highly oxidized mass of crystals and the micro- 
structure showed that the brittle eutectoid rich in tin had not 
been absorbed in the alpha solution but was prominently visible 



Standard Zinc-Bronze Test Bars 35 

and interspersed through the alpha constituent, hence the low 
tensile strength of the specimen. 

No. 422 was a barrel-shaped specimen, poured at a temperature 
of 1175° C (2147° F). It had a tensile strength of 32 000 pounds 
per square inch, an elongation in 2 inches of 11 per cent, and a 
reduction of area of 10 per cent. 

All of the above specimens were poiu-ed vertically in dry sand. 

No. 380 was a cast-to-size shape bar, poured at a temperature 
of 1100° C (2012° F). It had a tensile strength of 38 500 pounds 
per square inch, a yield point of 18000 pounds per square inch 
(by drop of beam), an elongation in 2 inches of 15.5 per cent, 
and a reduction of area of 11.5 per cent. Its microstructm-e 
showed an almost complete absorption of the tin-rich eutectoid 
in the alpha solution. It was poured vertically in dry sand. 

No. 19 was a cast-to-size shape bar, poiured at a temperature of 
1155° C (2111° F). It had a tensile strength of 48050 pounds 
per square inch, a yield point (by drop of beam) of 29 300 poimds 
per square inch, an elongation in 2 inches of 26 per cent, and a 
reduction of area of 20 per cent. The microstructure shows an 
almost perfect crystalline formation, with the tin-rich eutectoid 
almost completely absorbed in the alpha solution, and the den- 
dritic crystals uniformly distributed. It was poured fiat in green 
sand. 

No. 9 was a large size cylindrical bar, 2 inches in diameter, 
poured at a temperature of 1140° C (2084° F). It had a tensile 
strength of 17 000 poimds per square inch, an elastic limit of 
14 500 pounds per square inch, and an elongation in 2 inches of i 
per cent; reduction of area not recorded. The microstructure 
showed various holes which indicated inequality of shrinkage 
during cooling and the formation of two different solutions which 
had separated out in the slow cooling of the mass, which promoted 
the separation out of the tin-rich eutectoid and thus caused the 
low tensile strength of the specimen. It was poured fiat in green 
sand. 

VIII. HEAT-TREATED SPECIMENS 

The eighth series cast was devoted to tests of the action of 
annealing and cooling slowly, and annealing followed by quench- 
ing, in which the bars were submitted to temperatures of 500°, 



36 Technologic Papers oj the Bureau of Stayidards 

6cx)°, and 700^ C {932^, 1112^, and 1292^ F). The tensile strength 
was not materially affected by such treatment, but as to ductility 
there was great increase in elongation and corresponding reduction 
of area after submitting the samples to temperatures between 600 
and 700° C (1112'^ and 1292° F). Fig. 9 shows graphically the 
results of such treatment. In the case of annealed bars superior 
results were obtained both for sand cast and chill cast bars in 
the temperature range from 700 to 800° C (1292° to 1472° F). 
In the quenched bars, sand cast, superior results were obtained in 
the temperature range from 600° to 700° C (1112° to 1292° F). 
With annealed bars the results agree ver\* well with those obtained 
by H. S. and J. S. G. Primrose^* but with the results obtained 
from the quenched bars are not concordant with theirs. 

Xo. S02 (see Fig. 4) was heated to 700° C (1292° F) and held at 
this constant temperature for 30 minutes in a gas muffle furnace 
and then allowed to cool in the muffle for 24 hours to room tem- 
perature. The eutectoid, indicated in a few places by minute 
white cr}'stals, has been almost completely absorbed by the alpha 
or copper-rich constituent during the annealing. This in turn 
has recrv'stallized for a depth of re inch on the outside of the 
specimen. The large increase in the elongation and reduction 
of area is probably due to the absorbed solution of the eutectoid 
rather than to the recr}'stallization. 

Xo. 814 (see Fig. 4 for appearance of specimen after fracture) was 
poured flat in dr\- sand at a temperature of 1255° C (2291° F), 
cast-to-size shape, heated to 700° C (1292° F), held 30 minutes 
at that temperatiure, and cooled in furnace for 24 hours. The 
surface layer appeared to be well annealed, the annealing extend- 
ing in for a distance of r? inch. The microscopic examination 
shows the metal to have been completely recr}^stalhzed into the 
characteristic polyhedral cr\'stals of bronze. 

A study of the specimens submitted to heat treatment in com- 
parison with others which have not been subjected to such treat- 
ment show that due either to recr}*stallization or to the absorption 
of the beta formation by the alpha formation to form one soUd 
homogeneous solution there is a marked increase in the values of 

'♦ Jonr. Inst, ex Metals, 1913, p. it-. 



Standard Zinc-Bronze Test Bars 



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38 Techtwlogic Papers of the Bureau of Standards 

the elongation and the reduction of area without a change of any 
magnitude in the tensile strength. The removal or absorption of 
the brittle constituent seems to make the bars so treated much 
more uniform in their physical properties and would indicate by 
analog}^ that commercial castings themselves when submitted to 
such treatment, where such treatment is feasible, would be greatly 
improved in character. It is also s-urmised that such treatment 
would tend to protect such castings from hydraulic f ailiure by clos- 
ing the grain and lessening the interstices or voids between the 
cr\"Stals and by limiting the cr}^stallization to a form and compact- 
ing the mass of cr}"stals to an extent which would render them 
less susceptible to hydraulic failure. Experiments are to be con- 
ducted in the near future to test the truth of the hypothesis. 

IX. CHOICE OF TEST SPECIMENS 

In general, the test piece should correspond to the t^-pe of casting 
so as to be representative of the principal physical properties of 
the latter. 

Where it is desirable to obtain a knowledge of the physical prop- 
erties of a casting made in sand, the test bar should be cast in 
sand, at approximately the same time and from the same heat. 
Judging from the experiments made at the BuresLU it is possible 
to seciu-e specimens cast to size in sand and poured within a range 
of temperatures 'from 1120° to 1270° C (2048° to 2318° F), which 
will have a tensile strength of 37 000 pounds per square inch, an 
elongation of 15 per cent in 2 inches, and a reduction of area of 
14 per cent; and under regular foimdr}' practice, using similar 
materials and tmder similar conditions, such a test bar might be 
expected to have a tensile strength of 33 000 pounds per square 
inch, an elongation of 14 per cent in 2 inches, and a reduction 
of area of 13 per cent. But if a bar of similar character be sub- 
mitted to heat treatment — that is, heated at 600° C (1112° F) in 
a furnace for 30 minutes and quenched or allowed to cool in the 
furnace to normal temperatiu-e — it might be expected to show a 
tensile strength of 36 000 pounds per square inch, an elongation 
of 25 per cent in 2 inches, and a reduction of area of 24 per cent. 

The properties of a chiU bar are not those of the casting, but 
can be made to approach it ver\- closely by proper heat treatment; 



Standard Zinc-Bronze Test Bars 39 

but if the casting is to be made in a chill mold, the test bar corre- 
sponding to the same heat should also be made in a chill mold, 
preferably an open one, as in accord with ordinary foimdry prac- 
tice. The tensile strength of a chill-cast bar is not favorably 
influenced by heat treatment, but its ductility is considerably in- 
creased; so that if a chill-cast bar first be annealed or quenched 
at a temperature of about 600° C (1112° F) so as to restore its 
crystalline structtire to a state resembling that of the sand-cast 
specimen, there may be expected of it a tensile strength of 30 000 
pounds per square inch, an elongation of 16 per cent in 2 inches, 
and a reduction of area of 15 per cent, which closely approximates 
that of the sand cast-to-size bar not heat treated. The chill bar 
so treated may properly be referred to as a normalized chill bar. 
The heat treatment of such bars may be performed in a simple 
manner, by making use of a gas mttffle fiunace, in which the bars 
are placed and heated for 30 minutes to a temperatmre of 600° C 
(1112° F), measured by a thermocouple pyrometer, and either 
allowed to cool down in the furnace for 24 hours to normal tem- 
perature or immediately quenched. 

X. SUMMARY AND CONCLUSIONS 

1 . The best type of test bar where the metal is to be cast into 
sand is the cast-to-size shape. If the metal be poiured within the 
range 1120° to 1270° C (2048° to 2318° F), greater uniformity 
of tensile strength and ductility may be secured. 

2. It was anticipated at the beginning of these investigations 
that some one temperature would be found at which the best and 
most imiform results as to the two important physical properties — 
tensile strength and ductihty — ^would be obtained, but the series 
of tests showed that instead of one such temperature there was a 
zone of temperatures, viz, 1120° to 1270° C (2048'' to 2318° F), 
at which good results might be expected an37where within that 
favorable zone of poiu-ing temperatures. 

3. The kind of mold to be recommended is a dry-sand mold, 
which should be poiu-ed either flat or vertical. The first method 
is more economical of time and labor, but has, however, no su- 
periority over the vertical potuing in the matter of technical 
results. 



40 Technologic Papers of the Bureau of Standards 

4. The results of the chill and best sand cast test bar are of 
almost equal value if the chill bar be annealed at temperatures 
ranging from 500° to 700° C (932° to 1292° F), but otherwise the 
former is inferior to the sand cast-to-size shape as a representative 
test bar. 

The advantages of the chill bar are that there are no molding 
requirements and that it can be poured by unskilled labor. Its 
disadvantages are that it is expensive to machine into shape and 
size required for testing, and it gives results both as to tensile 
strength and ductility that are misleading. 

The advantages of the cast-to-size shape are that it is easy to 
mold and inexpensive to machine into the shape and size reqmred 
for testing. From the results obtained it is believed to be the 
form which should be adopted as standard for general foundry 
practice. 

5. Heat treatment is productive of increased ductility, and has 
a marked effect on the deformation produced by continuously 
increasing loads on specimens annealed in the temperature zone of 
600° to 700° C (1112° to 1292° F), which is in accord with the 
results of other observers. 

6. As an alternative for the sand-cast specimen, if one prefers 
to work with a chill-cast specimen, as is the case in some foundries, 
the results show for the few observations taken that when annealed 
between a temperature range of 500° to 700° C (932° to 1292° F) 
a normalized chill bar is secured which is comparable to the sand 
cast-to-size shape. The specimens annealed and quenched offer 
no appreciable superiority to the specimens annealed and cooled 
slov/ly. 

7. The average density of sand-cast specimens is about 8.58 and 
of chill cast about 8.6. 

8. The elastic limit of this zinc bronze varies from 15 000 to 
17 000 pounds per square inch. 

9. The cooling curves show that the melting point of the alloy 
is about 980° C (1796° F) and that there is a pronounced transition 
point at 780° C (1436° F) and another feeble one at 530° C (986° F). 

10. In Table 5 is recorded a summary of all the results obtained 
for physical properties arranged in terms of the type of casting. 



Standard Zitic-Bronze Test Bars 

TABLE 5 

General Averages of all the Observations Taken 



41 





Shape 


Tensile strength 


Elonga- 
tion in 
2 inches 




Number 
of bars. 


Pounds 

per 
square 

inch 


Kilogram 

per 
square 
centi- 
meter 


Reduc- 
tion of 
area 


217 
161 
95 


Cast to size 

Cylindrical 

Barrel 


37 869 
32 832 
32 029 


2662 
2308 
2252 
2557 
2685 
2798 
2190 


Per cent 
15.5 
14.9 
14.2 
6.3 
27.0 
26.9 
17.0 


Per cent 
14.7 
13.9 
13 8 


88 


Chill cast 


36 512 

38 190 

39 810 
31 160 


6.9 


21 




26.0 


16 


Sand cast, auenclisd . 


24 2 


5 


Cliill '•^st, anrifinlpd 


16 7 









In all of the above it is to be borne in mind that only pure cop- 
per, tin, and zinc have been used, and the restdts obtained are 
characteristic of variations in f oimdry practice on virgin metal only 
once or twice melted except as noted in the text. The presence of 
scrap containing small amounts of lead, manganese, iron, and other 
ingredients which might influence the results of the tests with a 
standard bar are thus eliminated. 

II. As a supplement to these experiments, there is being car- 
ried out under the auspices of the advisory committee on nonfer- 
rous metals to the Bureau of Standards a series of comparative 
foundry tests by several members of this committee in cooperation 
with the Bureau of Standards, using identically prepared metal 
ingots, prepared by one of the committee, to determine what uni- 
formity of results may be obtained by different founders in the 
preparation of test specimens of this alloy as judged by the usual 
physical tests. 

For the work above described, the physical determinations were 
made by R. P. Devries and E. L. Lasier; the microscopic exami- 
nations by H. S. Rawdon; and the thermal analysis by J. J. Crowe, 
and to all of these gentlemen the author is deeply indebted for 
their interest and cooperation in this investigation. The foundry 
operations were carried out in the Pittsburgh laboratories of the 
Bureau of Standards, and here also the very considerable labor 
of preparing the test pieces was executed. 

Washington, March 27, 191 5. 



APPENDIXES 



Appendix A.— SUGGESTED SPECIFICATIONS FOR THE PREP- 
ARATION OF STANDARD ZINC BRONZE (Cu 88, Sn 10, Zn 2) 
TEST BARS 

MIXING AND MELTING 

Weigh out the required quantities of electrolytic copper, Straits tin, and Horsehead 
zinc. 

Cover the bottom of the crucible with pieces of charcoal about the size of a hickory- 
nut. Put in all the copper the crucible will hold, and cover the surface with small 
pieces of charcoal (no dust). Should the crucible not have the capacity to contain 
aUof the copper charge, heat the charge contained until one or two of the pieces of 
copper are melted, crowd the remainder down into the molten mass, and add the 
remaining copper, bring the whole charge into a molten state, then add the tin, stirring 
the w^hole thoroughly with an Acheson graphite rod stirrer, and after previously heating 
the zinc, add the amount of zinc weighed out and stir thoroughly. Keep the pot 
covered with a graphite cover during the melting period. After adding the zinc 
allow the whole charge to come to a good melting heat, denoted by the play of zinc 
flames over the surface or at the edge of the surface adjoining the crucible. Remove 
the pot from the fire and take the pouring temperattue with a tliermocouple. Pour 
the metal at any temperature between 1120° and 1270° C (2048° and 2318° F), but 
note the temperattire at which the bars are poured. Use P-Pt Ir or P-Pt Rh 
(10 per cent) thermocouple protected by a fused silica tube or an equivalent pyrometric 
method of determining the pouring temperature. When using platinum-rhodium 
couples add five-tenths of the temperature recorded at the cold junction to tempera- 
ture reading of milli voltmeter for the corrected pouring temperature, or 0.75 when 
using a platinum-iridium couple. 

MOLDING 

Use any good molding sand equal to Albany No. 2. If green sand be used, the 
moisture content should not exceed 12 per cent. If dry sand be used, face the mold 
with a layer of sand equal in fineness to No. 00 Crescent sand, tempered with molasses 
water. WHien the mold is finished spray the molds and nmners and pouring gate 
with molasses water, and dry each flask before the drying oven until the deepest 
pattern or feeding gate is completely dry. Allow the mold to cool to room temperature 
before being poured. Pick the vent holes with a fine needle in every one of the 
bulb reservoir feeders. 

GATING 

Put not less than three patterns of cast-to-size shape in each flask, gated as shown in 
Fig. I, c. 
42 



Standard Zinc-Bronze Test Bars 



43 



POURING 

Skim the pot carefully before pottring and hold back dross and scoriae with the 
skimmer as much as possible before pouring. Pour steadily with a stream that will 
just about half fill the runner, and not large enough to choke the bore of the runner, so 
as to allow the bulk of the gases carried down by the poining stream to escape upward 
through the space left in the runner and the pouring head. With every heat pour 
a wedge-shaped chill bar about 13 inches long by i% inches deep by i% inches wide 
on top. Always pour the chill bar nearly flat and from the same end. Each chill 
will make three test bars. Always begin the numbering of the chill bars from the 
pouring end. 

The size of the cast specimen in the rough should be 4% inches long, f^ inch diameter 
at center, and ^^/^e inch diameter at threaded ends. 

The size of the finished specimen should be, length over all, 4^ inches, length of 
center section 2 inches between gage points, size of threaded ends ^ inch diameter 
by I inch long, length of each fillet % inch, diameter of center section 0.505 inch. 

Appendix B.— COMPARISON WITH NAVY SPECIFICATIONS 

The Navy specifications call for a tensile strength of 30 000 pounds per square inch 
and an elongation of 15 per cent. A comparison of the tests of the zinc-bronze bars 
with these requirements are shown in the accompanying table. 

TABLE 6 



Percentage 
of bars 
meeting 

Navy 
require- 
ments 



Percentage 

of the 

remainder 

coming 

within 10 

per cent of 

same 



Percentage 
meeting 

only 
require- 
ments 
of tensile 
strength 



Cast in sand: 

Cast-to-size shape 

Cylindrical shape 

Barrel shape 

Cast-to-size shape annealed. 

Cast-to-size shape quenched 
Cast in chill mold: 

Chill bar 

Chill bar annealed 



53.0 
52.8 
42.1 
100.0 
100.0 

4.5 

100.0 



27.0 
27.7 
10.0 



1.0 



89.8 
73.3 
66.0 



If one considers the tensile strength alone, it will be found that all of the test 
shapes and methods might be considered satisfactory, but the requirements for 
elongation are so rigid that only when the bar or the casting to which it refers is 
submitted to heat treatment is one able to meet these requirements; also this treat- 
ment restores the metal to its natural state and true condition and thus develop fully 
the physical properties of which it is capable. 

If we study the effect of heat-treating test bars at the various temperatures of 500°, 
600°, 700°, and 800° C (932°, 1112°, 1292°, and 1472° F), for a period of 30 minutes, 
followed either by slow cooling in the muffle furnace to normal temperature or by 



44 Technologic Papers of the Bureau of Standards 

quenching in water, it is found that, whereas the tensile strength is not greatly 
increased, the elongation and reduction of area are increased to a remarkable extent. 
The range of temperature from 700° to 800° C (1292° to 1472° F) appears to be the 
most favorable in securing the greatest increase in such properties. 

In order to write a specification for the physical properties of a zinc bronze that 
would not be too difficult of fulfillment, without detriment to the public service, it 
is necessary to consider what may be reasonably expected of such a metal alloy as 
zinc bronze. It may readily be seen from the experimental work cited that a tensile 
strength of 30 000 pounds per square inch is not an unreasonable requirement. 

With reference to the elongation desired, it is found that 15 per cent is difficult 
to obtain for specimens that represent small castings, because such castings, owing to 
their small sectional area, receive an initial chill which materially lowers the amount 
of possible elongation, and for such a casting the cast-to-size shape, on account of its 
relatively small sectional area, is naturally a representative type of test bar and 
should be chosen, but it is open for consideration whether 15 per cent elongation 
for such a specimen is not too severe to be expected in the course of ordinary practice. 
In all probability 14 per cent elongation and 13 per cent reduction of area is all that 
may reasonably be expected for this alloy when the casting to be tested has a large 
sectional area in proportion to its mass. If the sample be poured within the normal 
range of temperatures — that is, from 1115° to 1260° C, inclusive (2039° to 2300° F) — 
the tensile strength of 30 000 pounds per square inch is not difficult to secure, and it 
is possible in a majority of cases to obtain an elongation of 15 per cent and a reduction 
of area of 14 per cent, and for such castings a cylindrical shape may be chosen. 

If a casting is to be chill cast, while it is ver>'- easy to secure a tensile strength of 
30 000 pounds per square inch, it is, on the other hand, impossible to secure an 
elongation of 15 per cent; in fact, an elongation of 4.5 per cent would be a fair equiva- 
lent for such a casting. This is because the initial stress is so great as to almost 
completely destroy the property of elongation. Now, if a chill bar is submitted to a 
proper heat treatment, either annealing or annealing and quenching as above 
described, it is quite possible to obtain and reasonable to expect an elongation of 
15 per cent. 

Hence from the above considerations it is fair to conclude that for small castings a 
tensile strength of 30 000 potmds per square inch with an elongation in 2 inches of 
14 per cent and a corresponding reduction of area of 13 per cent may be required 
when using a cast-to-size shape test bar; for large castings, 30 000 pounds per square 
inch tensile strength, 15 per cent elongation in 2 inches, reduction of area 14 per 
cent; for chill castings, untreated, 30 000 pounds per square inch tensile strength, 
elongation 4.5 per cent, reduction of area 4 per cent; and for chill castings, heat 
treated, the same tensile strength, an elongation of 15 per cent, and a reduction of 
area of 14 per cent. 

Appendix C— AMERICAN SOCIETY FOR TESTING MATERIALS 

PROPOSED STANDARD SPECIFICATIONS FOR THE ALLOY— COPPER 88 
PER CENT, TIN 10 PER CENT, ZINC 2 PER CENT 

I, (a) These specifications cover the alloy commercially known as Government 
bronze. Admiralty gun metal, gun metal, or 88-10-2 mixture, when used in castings. 

(6) It is recommended that this alloy shall not be used where castings are subjected 
to a temperature exceeding 260'' C (500° F). 



Standard Zinc-Bronze Test Bars 45 

I. MANUFACTURE 

2. The alloy may be made by any approved method. 

n. CHEMICAL PROPERTIES AND TESTS 

3. The alloy shall conform to the following requirements as to chemical composition: 

Per cent. 

Copper 87-89 

Tin 8-11 

Zinc 1-3 

4. (a) The sample for chemical analysis may be taken either by sawing, drilling, 
or milling the casting or tension test specimen and shall represent the average cross 
section of the piece. 

(6) The saw, drill, cutter, or other tool used shall be thoroughly cleaned. No 
lubricant shall be used in the operation and the sawdust or metal chips shall be care- 
fully treated with a magnet to remove any particles of iron derived from the tools. 

m. PHYSICAL PROPERTIES AND TESTS 

5. The alloy shall conform to the following minimum requirements as to tensile 
properties: 

Tensile strength, pounds per square inch 7,:^ 000 

Elongation in 2 inches, per cent 14 

6. (a) The test bars of the form and dimensions shown in Fig. i shall be an integral 
part of large castings, or cast separately in the case of small castings to represent a 
lot or melt, and shall be molded in a manner similar to the castings which they repre- 
sent. If the castings are heat treated, the test bars representing such castings shall 
be similarly heat treated. 

(6) The manufacturer and purchaser shall agree v/hether test bars can be attached 
to castings, on the location of the bars on the castings, on the castings to which bars 
are to be attached, and on the method of casting unattached bars. Unless otherwise 
agreed upon the minimum lot shall be considered as 500 pounds. 

(c) The tension test specimen, turned down from test bar shown in Fig. i, shall be 
of the form and dimensions shown in Fig. 2. 

Note. — Fig. i above is understood to be the same as Fig. i (a), p. 147 of Vol. XIV, 
Proceedings, 19 14, Part II: Fig. 2 standard 2 by ^ inch threaded specimen. 

7. (a) Two tension tests shall be made from each lot or melt. 

(&) If any test specimen shows defective machining or develops flaws, it may be 
discarded, in which case the manufacturer and the purchaser or his representative 
shall agree upon the selection of another specimen in its stead. 

IV. INVESTIGATION OF CLAIMS 

8. If the purchaser's tests show that the material does not conform to the require- 
ments specified in section 3 , the manuf actiurer shall have the opportunity to inspect 
the material and each party shall select a sample for retest. If the results do not agree, 
each shall select a sample to be final. The costs of retests shall be paid by the loser. 



Part II.— MICROSTRUCTURE 



I. INTRODUCTION 

NoTS. — The work herein described constitutes a part of the study of the properties 
of the standard zinc bronze (Cu 88, Sn lo, Zn 2), the preparation of which has been 
described in Part I. The specimens here used are referred to by the same numbers 
that are there assigned to them. 

The various properties of any heterogeneous material depend 
primarily upon the properties of the units of which it is composed 
and the method by which these constituents are joined together. 
Alloys are no exception to this rule. Most of the commercial 
alloys are fairly complex in their make-up and are composed of 
units, the different classes of which are often of widely differing 
properties. The fact that the units out of which the metal or 
alloy is built are generally of microscopic proportions often leads 
to their being overlooked and underestimated. Besides the con- 
stituents expected to be present, from a knowledge of the chemical 
composition, various inclusions or contaminations, not added 
intentionally, often have an important bearing on the properties 
of the alloy and its behavior in service. A knowledge of the 
physical structure of any alloy, the characteristic features of each 
class of structural units, and the method by which the final 
product is built up out of these is the best foimdation for a rational 
understanding of the properties of the alloy and its probable 
behavior under varying service conditions. 

II. THE EQUILIBRIUM DIAGRAM 

Any alloy is most conveniently and properly studied by refer- 
ring it to its place in the proper series and regarding it as one of a 
family rather than an isolated individual. By this means any 
change in properties resulting from changes in composition may 
be studied systematically rather than at haphazard. The so- 
called equilibrium diagram represents concisely the structures 

47 



48 Technologic Papers of the Bureau of Standards 

existing as a result of varying composition and the structural 
changes resulting from variations in temperature. For purposes 
of reference, a portion of the equilibrium diagram of the copper- 
tin alloys is reproduced. This is one of the most complex of the 
structural diagrams of the technical alloys, but fortunately the 
alloy under consideration lies well within the less complex part. 

In spite of the great amount of study the alloys of copper and 
tin have received, there is still no well-defined agreement as to 
many of the percentage compositions which mark the boundaries 
of the different structural fields ; the limit of the a solution varies 
from 5 per cent in Giolitti's ^^ diagram to 13 per cent in Shep- 
herd's ^^. The percentages shown are given by Gulliver " and 
represent conditions of equihbrium, obtained either by excessively 
slow cooling of the molten alloy or by reheating an alloy which 
solidified by cooHng at an ordinary rate. 

Any point of the diagram represents the structure of the alloy 
at the temperature corresponding to the composition represented 
by that point. Above the line ABC the diagram repre- 
sents the alloy in the molten state, below Aah the alloy 

is solid. Between the two, both solid and liqxiid exist, and in 
passing through the corresponding temperature range the alloy 
soUdifies or melts according to whether it is being heated or 
cooled. The diagram shows in a general way the effect of addi- 
tions of tin upon the melting point of copper. 

Alloys of a composition between the points A and a' consist of 
but one constituent — the a bronze, which is a solid solution of 
the two elements, the copper predominating; at a' (13 per cent 
Sn approximately) a second constituent appears and grows in 
amount as the tin is increased until at 28 per cent (approximately) 
the entire alloy consists of this second constituent. The prop- 
erties of alloys lying between a' and d will depend largely then 
upon the relative amounts of these two structural units. 

The addition of small quantities of zinc does not materially alter 
the structure of the alloy under consideration. The copper-rich 
alloys of copper and zinc — i. e., in the a field — are indistinguish- 

1^ Giolitti and Tavanti, Gazetta Chimie, 38, p. 209, 1908. 

IS Shepherd and Blough, Jour. Phy. Chemistry, p. 515, 1906. 

1' Gulliver. Metallic AUoys, p. 249. 



Standard Zinc-Bronze Test Bars 



49 



able microscopically ordinarily from the corresponding ones of the 
copper-tin series. In speaking of the microstructure of the copper- 







Fig. io. — Structural equilibrium diagram of copper and tin 

rich kalchoids, Hoyt^^ emphasizes the fact that the "micro- 
structure of the ternary alloys corresponds very closely to that of 

18 S. L. Hoyt, Jour. Inst. Metals. 1913. no. 2. vol. X. p. 235. 



50 Technologic Papers of the Bureau of Standards 

the two binary systems." The zinc aids materially in the prepa- 
ration of the alloy by its deoxidizing effect upon any oxides of tin 
or copper which may be present and causes the metal to be more 
fluid and thus flow more easily. Any excess of zinc present most 
probably exists in solution in the a constituent of copper and tin. 
Fig. 1 1 shows that portion of the copper-zinc diagram containing 
the alloys rich in copper. They are simpler than the copper-tin 
alloys; the a field extends over a much greater part of the series. 
If we adopt Gmllet's suggestion ^^ of the " fictitious value " of an 
element we may regard the small percentage of zinc added as 
equivalent in effect to a certain amount of tin in excess of what 
was actually added. Guillet has determined experimentally that 
tin has a ** coefficient of equivalence" of 2, with respect to zinc as 
unity. Using the method suggested by Guillet, the 88-10-2 alloy 
is equivalent in structiire to a copper- tin alloy containing 1 1 . i per 
cent tin. If this suggestion is followed, as an approximation, for the 
study of the ternary alloys of copper, tin, and zinc which are low in 
zinc, it is unnecessary to depend on the ternary diagram of the 
series. 

III. STRUCTURE OF THE CAST ALLOY 

The structure of the alloy after solidification and before receiving 
any treatment, thermal or mechanical, is represented in Fig. 13, 
.4 and B. The structure here shown is that of specimen 19, which 
was used for the determination of the melting range and the thermal 
critical points. For the determination of these it was cooled 
slowly in a Heraeus platinum furnace. The very slow cooling 
accoimts for the coarseness of the structtire and also shows the 
practical impossibility of obtaining a homogeneous alloy directly 
upon soUdification. 

The matrix of the alloy shown is the a solution of zinc and tin in 
copper and shows a fernUke or dendritic pattern. The cores of the 
individual branches or fingers are the portions very rich in copper 
and the tin content increases as the outer portion of the "fijiger" 
is reached ; in the angles between the interlocking branches are the 
angular inclusions of a eutectoid consisting of two solid solutions of 

'5 L. Guillet, Rev. de Metallurgie, 3, p. 243, 1906, or C. H. Desch, Metallography, p. 191. 



Bureau of Standards Technologic Paper No. 59 












g,^" 



^^.v>t^-iiZv^ 



y4, Specimen 19; 5X. Remelted and allowed to cool 
in the furnace. The difference in orientation of the 
dendritic structure marks out the various crystals 




B, Specimen 19; 100 X. The dark portions are the 
cores rich in copper; the light angular spots, the 
hard brittle eutectoid inclusions 








C, specimen 568; looX. The shattering of the eu- 
tectoid imder strain is illustrated 




D, Specimen 568; 250X. The cracks (black) in the 
eutectoid (white) are at right angles to the direc- 
tion of the strain 



Fig. 13. — Typical microstructure of cast zinc-bronze {88-10-2) 

\_A and B were etched with ammonium hydroxide containing hydrogen peroxide; C and D with ammoniacal copper- 
ammonium chloride] 



Standard Zinc-Bronze Test Bars 



51 



tin and copper (i. e., disregarding the zinc for the time) of different 
tin contents. This eutectoid or " bronzite," as it has been termed 
by Guertler, may be considered as analogous to pearlite in steel. 

Reference to Fig. 10 will make clear the process of solidification 
by which the structure shown is obtained. Assume an alloy of 5 



f^ 


\ 
\ 


^ 












"-< 


^^u/^^^ 

^ 
^ 

^ 


^i^'^ >-^ - 


L/OU'd 




eoo 






— N 


\\" 


"-^T2> 


/ 
/ 


Tffff 










\ 


f 
I 
1 

1 


tiCfi 




a. 






a^jt 


\ / 

\ f 


/3^r 


































f^O 








^(Ti 






- 






Lee *Xl 








Fig. II. — Strtwtural equilibriutn diagram of copper and zinc 

per cent tin in the molten condition, represented by the point m. 
As the liquid alloy cools, solid crystals will first separate out at a 
temperature of 1050° C (approximately) and be of the composi- 
tion — approximately i per cent tin. The composition of the 
melt from which these copper-rich crystals separates is represented 
by n. The separation of the copper-rich crystals will increase the 



52 Technologic Papers of the Bureau of Standards 

tin content of the liquid portion, its composition now being repre- 
sented by a point on the cur^^e AC just below n at n'. The com- 
position of the succeeding solid will be correspondingly richer in 
tin and be represented on the cur\^e at o'. At w' (900° C) the 
process of solidification is complete and the composition of the 
solid crystal will vary in tin content from center outward, as repre- 
sented by the portion of the curve o—m'. The process of diffusion 
which accompanies the precipitation of the cr^^stals tends to \npe 
out any abrupt changes in composition and only a gradual varia- 
tion results. The temperature range nm' (1050° to 900° C) is the 
"freezing range" of the alloy; it has no definite freezing point. 
Inasmuch as metallic soHd solutions do not usually crystalHze from 
a single center outward uniformly but grow in certain directions 
faster than in others, a treehke form results, the cross section of the 
branches of which show a varymg composition as explained. The 
central core, being the first to soUdify, is the portion richest in 
copper. Since crystallization begins at a great nimiber of points 
throughout the cooling liquid metal simultaneously, one such tree- 
like crystal ^vill result at each center of crystalHzation, the size and 
exterior form of each " tree " will depend upon the intereference with 
neighboring ones. The whole mass when soHd will consist then of 
crystals, each one of which may be compared in structure to a 
soHd "double-ended" pine tree. This is shown diagrammatically 
in Fig. 12. 

Below 900° C no further changes occur in the cr}7stals other than 
diffusion within the crystals, which tends to erase the dendritic 
pattern and render the structure more homogeneous. 

The changes which occiu: in the 88-10-2 alloy upon cooling are 
much more complex. The first part of the process of sohdification 
is similar to the alloy just described; in this case the sohd separat- 
ing first from the melt is richer in tin (approximately 3 per cent) 
than the case already discussed. At 790° C the alloy is nearly 
solid and consists of dendrites of varying composition (97 to 91 
per cent Cu) and a liquid containing 74.5 per cent copper. At this 
temperature a reaction occurs between this liquid and some of the 
already solidified portion to form a solid of 77.5 per cent copper 
content; this is the jS solid solution. This ^ constituent lies in 



Bureau of Standards Technologic Paper No. 59 





A, Specimen 524; 5X. Poured as a chill bar at 1285° C. 



B. Specimen 524; looX 




C, Specimen 437; 5X. Poured at 1175° C. as a bar- 
rel-shaped casting in sand 




D, Specimen 437; looX 



Fig. 14. — Microstructure as affected by the rate of cooling 
[A and C were etched with ferric chloride; B and D with ammonium hydroxide containing hydrogen peroxide] 



Standard Zinc-Bronze Test Bars 



53 



the angles between the interlocking fingers next to the tin-rich 
layers of the dendrites. From 790° C to 500° C, as the alloy, now 
solid, cools, no appreciable structural change other than diffusion 
occurs. At 500°, the ^ solution breaks up into an intimate mix- 
ture of the a and h solutions of copper and tin. The 6 solution 
is still richer in tin than is the jS. This break-up at 500° is anal- 
ogous to the formation of pearlite in steel. The solid solutions 
Of, jS, 5, etc., are probably not simple solutions of tin in copper of 
progressively increasing tin content but their theoretical nature 
is not of importance here. The 88-10-2 alloy, then, at ordinary 




Fig. 12. — Illustration of "pine-tree'^ or dendritic crystalline structure 

temperatures, directly after casting, consists of a mass of solid 
treelike crystals the branches of which constitute the a solution 
of tin and copper (plus the zinc) and show a variation in composi- 
tion from center outward. In the interstices between the 
"branches" are inclusions of "bronzite," a eutectoid consisting 
of a mechanical mixture of a and b solutions of copper and tin 
which resulted from the break up of a pre\dousIy existing con- 
stituent, the /3 solution of copper and tin. By increasing the 
tin content of the alloy the amount of "bronzite" increases at the 
expense of the a solution until at about 25.0 tin; there is no free a. 



54 Technologic Papers of the Bureau of Standards 

Beyond this point free 5 solution appears and at approximately 
28 per cent Sn the bronzite has disappeared and the alloy is all 
6 solution. 

IV. RELATION BETWEEN METHOD OF CASTING AND 
MICROSTRUCTURE 

From theoretical considerations, no essential change in the 
general microstructure should be expected to accompany different 
methods of preparation and casting of the alloy. Microscopical 
examination of some 140 specimens confirms this. The method 
of casting exerts its influence indirectly by determining the rate 
of cooling and so also the rate of cn/staUization. The rate of 
cooling will be affected by the size of the molding flask, the heat 
conductivity and capacity of the molding sand, the presence of 
relatively large quantities of lAetal in a feeding reser\^oir, the pour- 
ing temperature, the use of chill molds, etc. The term "coarse 
and fine crystals " has a double meaning with reference to this 
alloy, and to cast alloys in general possessing a dendritic structure. 
The size of the individual cr^^stals themselves may vary and so 
also may the size of the "mesh" of the inner dendritic structure 
of the cr>^stal. The two are found not to accompany each other, 
necessarily. The size of the crystals being decided by the number 
of "crystallization centers," it is to be expected that, ordinarily, 
a very slow cooling will result in large crystals and a coarse-meshed 
dendritic structure. Mechanical disturbances, however, may 
interfere so that crystallization begins at a great many centers 
and smaller crystals with a fairly coarse interior structure may 
result with approximately the same rate of cooling. (Compare 
Fig. 15, a and c.) 

Another important result of the influence of rate of cooling upon 
the structure is the amount of eutectoid which is dissolved by the 
a constituent. Theoretically it should be possible with alloys 
of the percentage of tin here considered to have, at the most, 
only traces of the eutectoid showing. Practically this never 
results in the cast alloy. By the use of chill molds, the soHdifica- 
tion and the subsequent cooling of the alloy may be so rapid that 
the eutectoid inversion may be partially obliterated, so that the 
jS constituent is retained, at least in the outer crust thus imparting 



Bureau of Standards Technologic Paper No. 59 





A , Specimen 740; 5 X . Poured at 1 2 2 s ° C. as a " cast- 
to-size" shape in greensand. 



B, Specimen 740; looX 




C, Specimen 531; sX. Poured at i2oo°-C. as a " cast- 
to-size " shape in greensand 



D, Specimen, 531; looX 



Fig. 15. — Micro structure as affected by the rate of cooling 
[A and C were etched with ferric chloride; B and D with ammonium hydroxide containing hydrogen peroxide] 



Standard Zinc-Bronze Test Bars 55 

different ph3^sical properties to the alloy. This condition, how- 
ever, was not detected with certainty in any of the samples 
examined. 

Figs. 14 and 15 illustrate the general structure of the alloys 
prepared by different methods and illustrate the above points. 

V. APPEARANCE OF SPECIMENS AFTER THE TENSION TEST 

The appearance of the specimen after being broken in the ten- 
sion test is quite indicative of the numerical results obtained in the 
test. Fig. 16 illustrates this characteristic appearance of "good" 
and "poor" bars. A knowledge of the microstructiure affords an 
explanation of the cause of this characteristic appearance. Fig. 
12 shows that the relative orientation of the interior dendritic 
structure of neighboring crystals follows no definite plan; this is also 
confirmed by microscopic observation of the specimens. Those 
cry^stals which are most favorably oriented will be stretched beyond 
the elastic limit first; these most probably are those whose prin- 
cipal axis of orientation coincides with the direction of the applied 
stress. The roughening or "crinkling" of the surface of the ten- 
sile bar shows that different crystals are affected unevenly. In 
other v/ords, the mechanical properties of the individual crystals 
are not the same in all directions. Vv^hen those crystals which are 
most favorably oriented begin to yield to the stress, thus changing 
the relative amount other crystals must bear, another set of crys- 
tals will reach their elastic limit in that particular direction and 
so in turn yield to the stress. 

Fig. 13, C and D, show how the different microconstituents 
behave under tension and what the real nature of each is. The 
a solution, after its "elastic limit" is exceeded, is plastically de- 
formed by the acting stress. The eutectoid is a hard brittle sub- 
stance and can only conform to this new condition by breaking 
transversely across and having the resulting small sections move 
bodily in the direction of the applied stress. If the eutectoid 
forms a practically continuous network, its brittleness and other 
properties will predominate in determining the physical properties 
of the alloy. 

The smooth unwrinkled bars are characteristic of low results 
in general. In this case the eutectoid is not found in a shattered 



56 



Technologic Papers of the Bureau of Statidards 



condition and the a constituent has not "flowed" under the ap- 
plied stress. The presence of inclusions of impurities has pre- 
vented this and caused the low results. This will be referred to 
in a later section. 

The microscopic examination of the specimen offers evidence 
of the suggestion made above that the crystals are affected im- 
equaily according to their orientation. Crystals were found which 
showed shattered eutectoid particles and the a matrix in spots 
has an appearance suggestive of flowing while immediately neigh- 
boring crystals apparently were unchanged. 



VI. CORRELATION OF MICROSTRUCTURE AND PHYSICAL 

PROPERTIES 

A large number of broken tensile bars were chosen at random 
for the examination of the microstructiu-e and its correlation with 
the physical properties. The possible sources of weakness in such 
bars may be classed under tw^o headings — (i) those due to weak- 
ness at the crystal boundaries, badly interlocking crystals, segre- 
gated impurities at the exterior of the crystals, etc. ; (2) weakness 
inherent within the crystal structure, which may be due to crystal 
size and orientation, inclusions of foreign materials, excessive or 
badly disseminated eutectoid, etc. 

TABLE 7 
Microscopic Examination 

[The specimens given were chosen at random from a collection of nearly looo test barsj 



Speci- 
men 



Condition of bar o 



Color of fracture 



Type of 
break b 



Microscopic 
examination c 



Physical 
properties * 



Finely crinkled. 



Rather brown. 



109 



263 



rSmooth with fine trans- 

1 verse cracks. 



Earthy yellow. 



[Intracrystal- 
I line. 



.do. 



Crinkled 



Bright yellow. 



{Eutectoid moderate in 
amount; dendrites 
medium mesh. 
{Dendrites fine mesh; 
eutectoid abundant; 
pits common. 
Pits common; trace of 
oxide film; eutec- 
toid abundant. 



33 800 



42 000 
16 000 



2a 5 

2ao 



6.S 
7.0 



23.0 
20.0 



o The descriptive terms used are illustrated by Fig. i6. 

& Intracrystalline, through the crystals; intercrystalline. betwee^i them. 

c Theterms "fine" and "coarse" as applied to the dendritic structvire are relative, Fig.i4,5and D.may 
be taken as standard. Fig. 14, D, also illustrates the usual amoiuit of eutectoid to be expected. 

d Recordedin order: Ultimate strength in povinds per square inchi elastic limit in pounds per square inch; 
per cent elongation in 2 inches; per cent reduction of area. 



Bureau of Standards Technologic Paper No. 59 




A, Specimen 36; 2X. Ultimate tensile strength, 
42,450 povmds per square inch; elongation in 2 
inches, 20.5 per cent; reduction of area, 20 per 
cent 











ipw 


iHJi 


Sm 


S 


^^ 


m 


iP 


w 


^p^l 


w 











B, Specimen 404; 2X. Ultimate tensile strength, 
39.550 pounds per square inch; elongation in 2 
inches, 20.5 per cent; reduction of area, 20.5 per 
cent 





C, Specimen 475; 2X. Ultimate tensile strength, 
19,800 pounds per square inch; elongation in 2 
inches, 3 per cent; reduction of area, 2 per cent 



D, Specimen 1023; 2X. Ultimate tensile strength, 
26,000 pounds per square inch; elongation in 2 
inches, 5.5 per cent; reduction of area, 7.6 per cent 



Fig. 16. — Surface appearance of tensile bars after rupture 



Standard Zinc-Bronze Test Bars 
TABLE 7— Continued 



57 



Speci- 
men 


Condition of bar 


Color of fracture 


Type of 
break 


Microscopic 
examination 


Physical 
properties 


285 


Finely crinkled.. 


prellow plus or- 
ange in de- 
pressions. 


llntercrystal- 
1 line. 


[No oxide films; eutec- 
1 toid shattered. 

Coarse mesh for den- 


1 41 670 21. S 
J 19 000 21. 


301 


f Smooth with fine trans- 
1 verse cracks. 

Slight roughening of 


[Yellowish gray.. 


{....do 


drites; eutectoid 
abundant; oxide 


24 370 7.5 
15 000 7. 


312 


the surface; fracture 
is through three crys- 
tals of nearly same 
orientation. 


Yellowplusalit- 
1 tie gray. 


llntracrystal- 
1 Ihie. 


Like No. 314, with less 
eutectoid. 

Eutectoid abundant 


1 32 200 15.0 
J 14 620 11. 5 




rXrace of fine roughen - 


Yellow plus 


1 


and in relatively 




314 


\ mg with some trans- 


many bright 


l do 


large masses; pits 


30 900 7.0 




1 verse cracks. 


gray spots. 




abundant; some ox- 
ide fiUns found. 
[Medium mesh; eutec- 


17 500 4.0 


327 


Finely crinkled 


Dull yellow 


do 


toid, small and scat- 
tered; pits few; no 
oxide. 


40 902 19. 
20 000 16.0 


342 


Smooth 


Grayish yellow.. 


[Crystals too 
1 small to de- 
1 cide. 


Poor mterlockmg of 
dendrites; eutectoid 
forms a network; 


23 100 .... 






15 550 4.0 






"* 


oxide films present. 




372 


Just a trace of crinkling ; 
fine transverse 
cracks. 


...-do 


flntercrystal- 
1 line. 


f Dendrites, large mesh; 
many pits and oxide 
I films. 
Dendrites, coarse 


25 750 7. 5 
16 700 7. 


374 


Trace of crmkling of 
surface. 


Orange plus 
brown. 


Intracrystal- 
Ihie. 


mesh; eutectoid 
plentiful; pits plen- 
tihil; a few oxide 
films. 


37 120 15.0 
16 200 9.0 


404 


Much roughened 


Yellow 


do 


[Dendrites, large mesh; 
mesh; trace of oxide 
films; pits plentiful. 


39 550 20.5 








20.5 










Dendrites, large mesh ; 




407 








eutectoid in small 
particles; few pits; 


42 500 23. 5 










25.T 










no oxide films. 




411 


Smooth 


Gray plus a little 
yeUow. 


Too finely 
crystalline to 
be deter- 
mined. 


Dendrites are of a very 
fine mesh and much 
branched; eutectoid 
abtmdant; no oxide 
films; few pits. 


36 200 5.0 






20 000 4.0 



58 



Technologic Papers of the Bureati of Standards 
TABLE 7— Continued 



Speci- 
men 


Condition of bar 


Color of fracture 


Type of 
break 


Microscopic 
examination 


Physical 
properties 




fSurface is slightly 
1 roughened. 


YeUow plus a lit- 


Intracrystal- 


V 

Dendrites of large 


1 31 200 10. 
[16 500 10.5 


416 


tle gray. 


line. 
Intracrystal- 


mesh; pits common; 
some oxide films. 






i 


line through 






418 


[Very slightly rough- 
1 ened. 


] 
YeUow with dis- 
tinct tinge of 


a crystal ex- 
tending one- 
third the di- 


Dendrites, coarse 
mesh; eutectoid 
abundant; pits plen- 


30 500 12.0 
10.0 






gray. 


ameter of 
bar. 
Intracrystal- 
line through 
a large crys- 


tiful; no oxide films. 
Dendrites, coarse 




425 


Noticeably roughened.. 


Yellow 


tal occupy- 
ing two - 
thirds di- 
ameter of 
bar. 


mesh; eutectoid not 
abtmdant; some pits 
and oxide films, but 
not abundant. 


32 800 13. 5 
23 800 12. 5 


431 


fvSmooth, with some 
1 transverse cracks. 


I 


Intracrystal- 
line. 


Many pits; some ox- 
ide films. 


26 900 7. 5 
18 500 5.7 




J 










Dendrites, medium 




439 


Much crinkled 




do 


mesh; eutectoid 
abundant; no oside 


44 500 24. 










films; pits not com- 


14.0 










mon. 




445 


Finely crinkled 


jYellow with a 
1 tinge of gray. 

Yellow with or- 


1-- 


jTraces of oxide fihns 
1 in spots. 

Dendrites, medium 
mesh; eutectoid 


r 41 000 17.0 
1 15.8 


447 


Smooth 


1 ange, also has \ 

prominent 
I gray spots. | 


Indetermin- 
able. 


shattered in some 
cr>-tals; some pits 
and oxide fihns 


33 400 15.0 






16 300 16.7 










. found. 












Dendrites, coarse 




471 


do 


jYellow with a 
1 gray tint. 


Intracrystal- 
line. 


mesh; pits common 
with some oxide 


26 000 8. 5 






■ 16 000 7. 5 










films. 




472 


jSmooth, with some 
1 transverse cracks. 


[Bright orange 
and brown 


Intercrystal- 


Eutectoid plentiful and 
in large particles; j 


21 000 3.5 




1 with some gray 


line. 


pits common; oxide 


15 600 2.5 






I spots. 




films very bad. 




474 


do 


do 


....do 


Oxide films very bad . . 


J 17 300 2. 
114 800 1.5 


475 


pFaint traces oi roughen- 
ing with transverse 
cracks. 


[YeUow with a 
j tinge of gray. 


I n t r acrystal- 
line. 


Oxide films are abun- 
dant. 


f 19 800 3.0 
i 15 000 2ja 



Standard Zinc-Bronze Test Bars 
TABLE 7— Continued 



59 



Speci- 
men 


Condition of bar 


Color of fracture 


Type of 
break 


Microscopic 
examination 


Physical 
properties 










Oxide films in inter- 








Dirty yellow 




crystalline bounda- 




490 


Smooth 


with brown 
areas and gray 


Intercrystal- 
line. 


ries, associated with 
pits; eutectoid in 


14 000 1. 




4 500 .5 






I spots. 




large particles; mesh 
. of dendrites coarse. 
Dendrites, fine mesh; 




494 


Very slightly crinkled.. 


Grayish yellow . . 




eutectoid, in fine 
particles; no oxide 
films; pits not com- 


41 400 10. 5 






3 000 10. 5 








1 


mon. 










1 

! 


Dendrites, large 




555 


Somewhat crinkled.... 


nreUow with a 
1 tinge of gray. 


Intracrystal- 
line. 


mesh; eutectoid not 
abundant; few pits; 
trace of oxide films. 
Dendrites, large 
mesh; eutectoid 


36 900 19. 5 
17.5 


568 


do 


I>ull yellow 


do 


shattered in some 
crystals; pits abun- 
dant; some oxide 
fihns present. 
Dendrites, large 


35 000 21.0 
19.0 


571 


Roughened 


Yellow 


do 


mesh; eutectoid 
abundant; some ox- 


30 500 13.0 






16.5 










ide films. 




573 


Roughened consider- 
1 ably. 


Dirty yellow 


do 

Intracrystal- 
j line through 


[Like 568, except eutec- 
toid was not foimd 
1 shattered. 

Dendrites, coarse 


38 500 20.0 
16.9 


603 


Roughened 




a crystal oc- j 
cupying two- i 


mesh; eutectoid not 
very abundant; some 


37 200 23.5 









22. 3 








thirds the i 


pits and a trace of 










diameter of j 


oxide fihns. 










the bar. , 










[Orange and 


1 


Dendrites, coarse 




613 


JSmooth, with many 
1 transverse cracks. 


dark brown 


Intracrystal- 1 


mesh; eutectoid 


24 100 9.0 
7.0 


with many 
gray specks. 


line. 


plentiful; pits and 
oxide films rather I 










abundant. 




619 


Smooth 




Intercrystal- 
llne. 


Oxide films and pits 
abimdant. 


r 20 705 3. 
[ 3-8 
















jD end rites, coarse 




621 


do 




Intracrystal- 
Une. 

j 


mesh; eutectoid { 
not very plentiful; , 


23 750 5.0 








4. 5 








pits and oxide films { 










1 
1 


fairly abundant. | 





6o Technologic Papers of the Bureau of Standards 

TABLE 7— Continued 



Speci- 
men 



Condition of bar 



Color of fracture 



Type of 
break 



Microscopic 
esamination 



Physical 
properties 



824 



831 



Fine crinkling. 



{Considerably rough- 
ened (annealed at 
700° C). 



rery slightly rough- 
ened (annealed at 
600° C). 



Very rough (heated to 
700° C and quenched 
in water). 

Very rough (annealed 
at 800° C). 

Very rough (annealed 
at 700" C). 



Finely crinkled . 



1009 



1014 



Roughened at one end. 



Somewhat roughened. 



Smooth. 



.Steel gray with 
flecks of yel- 
low. 



Intracrj'stal- 
line. 



.do. 



.do. 



.do. 



.do. 



[Yellow with a 
i little gray. 



Grayish yellow . 



.do. 



nrellow with 
I trace of brown 
[ in spots. 



Orange brown 
with grayish 
yellow in spots. 



...do.. 



...do.. 



Dendrites, coarse 
mesh; eu tec to id 
very abundant and 
forms a much- 
branched network; j 
oxides rare. 
[Dendrites gone; eu- 
I tectoid nearly ab- 
sorbed; small pits; 
I no films. 

fDendrites still show 
faintly; traces of eu- 
tectoid remain; 
many pits and oxide 
films. 

Trace of dendrites re- 
main; eutectoid ab- 
sorbed; some pits 
show. 

Dendritic pattern en- 
tirely gone; eutec- 
toid absorbed; a few 
very tiny pits. 

Like No. 836; also a 
faint trace of oxide 
film was foimd. 

Eutectoid not very 
abundant; den- 
drites coarse; some 
oxide films are 
present. 

Dendrites, fine mesh; 
eutectoid plentiful; 
pits and fine oxide 
films. 

Eutectoid not very 
abundant and in fine 
particles; some pits 
and a trace of oxide 
films. 

Dendrites, very fine 
mesh; eutectoid 
abundant; some ox- 
ide films. 



36 300 .... 
5.7 

45 500 36. 
30.7 

34 000 22.0 
29. 

40 000 37.0 
32.4 

47 000 40. 
34.0 

48 500 42. 
19.0 

35 600 19. 5 
16.9 

32 100 7.0 
8.4 

35 000 10. 5 
19.0 

26 000 5. 5 
7.6 



Bureau of Standards Technologic Paper No, 59 




A, Specimen 285; 3X 



B, Specimen 571; 3X. The large crystal extendmg 
nearly across the bar most probably determined 
the position of the break 




C, Specimen 312; 3X. The three crystals of nearly the 
same orientation act similarly to a single crystal 



D, Specimen 301 



Fig. 17. — Types of fracture: Intercrystalline and intracrystalline 
A, B, and C, intracrystalline; D, intercrystalline 



Stattdard Zinc-Bronze Test Bars 6i 

1. TYPE OF FRACTURE 

An interesting and important point is the determination whether, 
dming tension, the break finally occurs between or through the 
crystals. These two types of break may be termed intercrystal- 
line and intracrystalline, respectively. To determine this point 
the two parts of a good many broken tensile specimens were sol- 
dered accurately together and a longitudinal section cut through 
the soldered portion of each bar. The cut face was pohshed and 
etched to reveal the macroscopic crystalline structiure. The 
results are summarized in the preceding table. Fig. 17 illus- 
trates the appearance of several such bars and show clearly that 
the prevailing type of break is the intracrystalline one rather than 
a simple pulling apart of the crystals, as might be supposed. 
Rosenhain ^^ has aheady called attention to the fact that this tjqpe 
of break is characteristic of metals broken at ordinary tempera- 
tures. Of 44 bars examined the following results were obtained. 



With intracrystalline break. 
With intercrystalllne break. 
Indeterminable 

Total 



Number 



Per cent 



79.6 
13.6 
6.8 



100.0 



This is true of many of the specimens which remained smooth 
after breaking and which gave evidence of very low ductility, thus 
showing that even in such cases the region of the crystal boimd- 
aries is stronger than the interior of the crystals themselves. The 
specimens giving an intercrystalllne break were those showing the 
worst mechanical features of all those examined. 

2. COARSENESS OF THE DENDRITES 

The results obtained indicate that the size of the mesh of the 
dendritic structure is relatively a minor factor in determining the 
physical properties of the alloy. High tensile results were noted 
with specimens in which the interior crystalline structure was 

20 Jour. Inst, of Metals, 1913, 2, x. p. 119. "The Intercrj'stalliiie Cohesion of Metals." 



62 Technologic Papers of the Bureau of Standards 

relatively coarse, e. g., specimen 407. With very pure material, 
the coarseness of the dendritic stnictm-e undoubtedly will be an 
important factor in determining the physical properties of the 
alloy, but with commercial material other factors overshadow 
this one as shown by the results obtained in this series. 

3. EUTECTOID 

As is to be expected, any decided increase in the amount of a 
brittle constituent like the eutectoid found in this alloy will entail 
decided changes in the physical properties. In dealing with any 
series, however, in which the composition is fixed within rather 
narrow limits, as is the case here, the distribution and arrangement 
of this constituent rather than accidental variations in the amount 
are more important in determining the properties of the alloy. 
Two specimens in the series examined, Nos. 411 and 640, showed 
no decided bad feattu-es so far as inclusions, etc., are concerned. 
The dendrites, however, instead of forming the usual interlocking 
t3rpe with the eutectoid forming isolated masses in the angles of 
the branches, are of a more open form and the eutectoid forms a 
nearly continuous network enveloping the branches. Both of 
these specimens broke with a decided gray fracture and though 
the ultimate strength was near the average the specimen showed 
the characteristics of brittleness to be expected. This type of 
structure, however, was the exception rather than the rule in the 
observed series. Fig. 18, Z), illustrates this condition. 

4. SIZE AND ORIENTATION OF CRYSTALS 

In otherwise good and clean metal, the size and orientation of 
neighboring crystals will determine largely the properties of the 
materials. Specimens 425 and 603, while showing a fair structure 
otherwise, broke at a point where one crystal extended nearly 
two-thirds the diameter of the bar. The properties of this crystal 
then determined almost entirely the behavior of the whole bar; 
Fig. 16, B, illustrates this condition. Several adjacent crystals 
having nearly the same orientation will act in a manner very simi- 
lar to a single large one; such a specimen is illustrated in Fig. 16, C. 
Such cases are apparently imavoidable and must be expected to 
occm* in castings, as ordinarily prepared. 



Bureau of Standards Technologic Paper No. 59 





/I, Specimen 475; looX. The dark network is the 
film of oxide 



B, Specimen 475; 2soX 





C, Specimen 613; 2soX £>, Specimen 411; looX 

Fig. 18. — Microstructure of badly contaminated metal 

A, B, and C show the appearance of the oxide films; D illustrates the open dendritic mesh (dark) with a fairly continuous 
eutectoid network (light). [All were etched with ammonium hydroxide containing hydrogen peroxide] 



Standard Zinc-Bronze Test Bars 63 

5. OXIDE FILMS AND PITS 

The one predominating cause of low results, as illustrated by 
the series examined, is the presence of oxide inclusions. In speak- 
ing of this point Law ^^ says: 

So important is the deoxidation of metals and alloys that it would probably be 
no exaggeration to say that when the history of the alloy industry comes to be 
written, the record of progress during the past 20 years will be summed up in the 
words "the use of deoxidizers. " 

In order to check out the possible occiurence of iron as an 
impurity in some of the bars which gave especially low results a 
qualitative test was made. A 5-gram sample of specimen 474, 
one of the poorest and most brittle bars examined, was used. No 
detectable color was obtained with the potassium sulphocyanide 
test. 

The form in which an impvtrity occurs, rather than its actual 
amount, largely determines the deleterious effect produced. The 
work of Heyn and Bauer ^^ has shown that in the copper-tin 
alloys stannic oxide inclusions occur in the form of an intersecting 
network. Similar networks were found in many of the samples 
examined. Fig. 18, A, B, and C, shows their appearance. In 
many of these networks dove-gray inclusions were f oiuid. Most of 
them, however, were filled with a pulverulent form of oxide. 
The oxide is really in the form of films of which the network 
observed is the cross-sectional view. These films are found 
within the dendritic structure of the crystals, in the tin-rich 
portion, or in that portion solidifying toward the close of the solid- 
ification period. Consequently, they are found in conjunction 
with the eutectoid, which is often noticed to form a continuation 
of the film. Thus, the bad effects of each are enhanced. In 
addition to the oxide films, most of the specimens show porosity 
to a greater or less extent. In many cases these pores are filled 
with the same pulverulent form of oxide found in the films. These 
pits are usually associated with the films in case the latter are 
present. 

An inspection of the results given in Table 7 shows that of those 
specimens having an ultimate tensile stength of 30000 or less, 

** E. F. I*aw, Jour. Inst, of Metals, 1912, No. 2, p. 222. 

** Zdt. f tir Anorg. Chemie, 1905, 45, p. 52; E. Heyn and O. Bauer, "Kupper, Zinn, und Sauerstoff." 



64 Technologic Papers of the Bureau of Standards 

13 in numoer, all show numerous oxide films and pits. Of the 
12 specimens having an ultimate strength of from 30 000 to 35 000, 
10 show evidence of these films of oxide. The presence of oxides 
of tin and zinc in the form of pits and films may be considered, 
then, as a predominating factor in the cause of weakness in cast 
bronze. The color of the freshly broken fracture suggests that 
often cuprous oxide may be present. Of the two forms, the films 
are much more serious in the results than are the pits and blow- 
holes. Samples in which the oxide occurs almost exclusively in 
relatively large masses rather than thin, wide-spreading films, 
while possessing a lower ultimate strength and elastic limit than 
sound metal, show evidence of higher ductility, and on the whole 
appear as a much better grade of material than those in which 
the films predominate. Specimen 374 is an example of this. 

The question of the solubility of oxides in metals is of prime 
importance in the consideration of means to be taken for their elim- 
ination. Law, in the reference already cited, states that with the 
exception of cuprous oxide, the solubility of which in copper is 
well known, other oxides do not dissolve to any appreciable 
extent in the molten alloy. Heyn and Bauer found in their work 
on stannic oxide that it is not soluble in the alloy. The forms in 
which the oxides were always found to occur in the samples 
examined, either in pits or films, is evidence that they are held in 
mechanical suspension rather than in true solution. 

The work of Heyn and Bauer, already referred to, has shown 
that a covering of charcoal is not a sufiicient protection against 
oxidation for copper- tin alloys. The results obtained here suggest 
that zinc as a deoxidizer is not to be depended upon without more 
elaborate precautions than can usually be taken in the ordinary 
course of brass foundry work. 

6. CONCLUSION 

The general conclusion from the study of the microstructure is 
that the presence of oxides is a much more potent source of me- 
chanical weakness of the alloy in its cast condition than any of 
tlie other causes enumerated. From the standpoint of micro- 
structure, the variations in methods of casting, potudng tempera- 



Bureau of Standards Technologic Paper No. 59 





A, Specimen 41S; 5X. The right edge shows a trace 
of the peripheral layer of fine recrystallized metal 
due to the surface distortion of the tensile bar 
while being machined; heated for 30 minutes at 
700° C. and cooled in the furnace. The macroscopic 
crystalline appearance of the greater part of the 
bar has been unchanged 



B, Specimen 418; looX. From the central portion 
of A. The eutectoid. has been nearly absorbed by 
the matrix; the dendritic pattern has been par- 
tially erased 





C, Specimen 41S; looX. Part of the recr^^stallized 
peripheral layer of A 



D, Specimen 451; 100 X. From the outer portion 
of the tensile bar where the structure was distorted 
by machining; heated to 860° C, dropped to 700°, 
held 30 minutes and then cooled in furnace 



Fig. 19. — Micro structure as affected by heat treattnent 

[A, C, and D were etched with ferric chloride, B with ammonium hydroxide containing hydrogen peroxide] 



Statidard Zinc-Bronze Test Bars 65 

ture, etc., are to be regarded primarily as means for the production 
of sound oxide-free material and to confer no mysterious proper- 
ties upon the alloy. The frequent occurrence of oxides in this 
series, prepared imder careful supervision and using precautions 
for avoiding such contaminations, suggests the abundance of such 
inclusions in similar alloys as prepared commercially. The failure 
of such cast alloys for many purposes is most probably to be 
ascribed to the presence of oxide films rather than to any other 
cause. 

VII. MICROSTRUCTURE AS RELATED TO HEAT TREATMENT 

As recorded in the microstructtire, the effect of annealing the 
alloy will be revealed by the disappearance of the interior dendritic 
structure of the crystals, the solution of the eutectoid in the a 
matrix, and the recrystallization of the alloy into the well-known 
polyhedral forms so characteristic of annealed brass and bronze 
high in copper.^^ Heating, followed by quenching, will retain the 
alloy in some intermediate stage depending on the temperature, 
time of heating, etc. The quenching of bronzes higher in tin may 
involve the retention of the ^ solution which exists above 500° C 
(Fig. 10). This, however, may be disregarded here. Fig. 19, 
A, B, C, and D, shows the effect of heat treatment upon the 
structvire. * 

The changes observed in the properties of heat-treated specimens 
(Table 7, Nos. 819, 824, 831, 836, 838) are most probably to be 
attributed to the disappearance of the nonhomogeneity of the 
alloy by the intracrystalline changes, i. e., by the erasure of the 
dendritic structural pattern, and the absorption of the eutectoid 
by the a matrix. This hard and brittle constituent, existing 
in a multitude of tiny inclusions throughout the alloy, almost 
entirely disappears. It is to be inferred from the examination 
of fractured test bars (Sec. VI, i) that it is the intracrystalline 
changes upon anneaHng rather than those at the botmdaries that 
most affect the properties of the metal as a whole. 

23 This is true only in case the metal has been " cold worked " and hence distorted as is true here for the 
surface layer of the test bars. Unworked castings will show no " recrystallization ' ' upon annealing. 



66 Technologic Papers of the Bureau of Standards 

In speaking of the effect of heat treating this alloy, Primrose ^* 
mentions the growth of the original crystals and the coarsening 
of the dendritic pattern upon annealing. In none of the heat 
treated specimens examined was any evidence of an increase of 
crystal size fomid. As the dendritic structm-e disappears upon 
heating, the copper-rich cores spread by diffusion and the rather 
sharp outlines of this pattern disappear. Thus, the whole struc- 
ture appears coarser than before but it should not be considered 
as analogous or similar in properties to a dendritic structure 
which, directly after casting, is as course as this heat-treated one 
now appears to be. 

The effect of heat treatment upon badly oxidized metal is im- 
portant. The results obtained for specimen 824 illustrate that 
the bad effects upon the tensile properties can be partially elimi- 
nated; this sample, though badly pitted and showing oxide films, 
gave a fair ultimate tensile strength and high ductility. The 
heat treatment of this alloy was carried out with a different aim 
in view. However, the results suggest that a series of heat treat- 
ments of badly oxidized specimens with the special purpose of 
determining whether it is possible to convert the injurious films 
of oxide into other less objectionable forms would be valuable. 

^III ETCHING OF SPECIMENS 

The structure of the alloy is revealed by appropriate etching. 
The oxide pits and films are seen before etching, though they are 
most clearly revealed after the surface film of metal due to the 
poUshing is dissolved off by the etching fluid. For showing the 
macroscopic crystalline appearance directly after casting, a freshly 
prepared saturated alcoholic solution of ferric chloride is very 
suitable. If the etched specimen, after being washed in alcohol 
and dried, is coated with a thin layer of clear shellac, the con- 
trast between the crystals is greatly increased. This is valuable 
for photographic purposes. To reveal the dendritic structure of 
the crystals, a i-i (approximate) solution of ammonium hydrox- 
ide, to which one or two drops of hydrogen peroxide are added 
w^hile the specimen is immersed, may be used. The copper-rich 

-'^Jour. InsT. Metals, 1913, No. i. Vol. II, p. 158. 



Standard Zmc-Bronze Test Bars 67 

portion, or cores, are attacked most strongly and dissolved so that 
the tin-rich portions appear in relief. The eutectoid is but little 
affected and shows in good contrast against the etched background. 
The same structure is shown by the use of an alkaline solution of 
copper ammonium chloride. In this case the surface is not etched 
so deeply. An acid aqueous solution of ferric chloride (5 per cent 
solution acidified with i per cent of hydrochloric acid) is often 
used. This darkens the a matrix so that the eutectoid stands 
out in bold contrast. It has the disadvantage of not clearly 
showing the dendritic structure of the matrix. 

IX. SUMMARY 

(a) The addition of the small percentage of zinc does not affect 
the theoretical microstructure of the alloy. 

(6) The method of casting, pouring temperature, etc., affect the 
structure only indirectly by influencing the rate of cooling, amount 
and distribution of "inclosures," etc. 

(c) The microstructure offers an explanation for the charac- 
teristic appearance of the tensile bars after testing. 

(d) Of the various microstructural features affecting the physi- 
cal properties, oxide films must be considered to exert the greatest 
influence, by far. 

(e) The changes of microstructure accompanying annealing are 
explained and illustrated. 

The very efficient help given by Arthur C. McCabe in the prep- 
aration of the man}^ metallographic ' sections necessary for ex- 
amination in this study is much appreciated and is here 
acknowledged. 

Washington, March 25, 191 5. 










4- ii ?^ ^■<'. 
ft 



(Ccmtinued from, p. 3 oi cover.; 
34. Detennination of Ammonia in Illtiminating Gas {23 pp.). 
2^. Combustion Method for the Direct Determination of Rubber (11 pp, j. 

36. Industrial Gas Calorimetry (150 pp.). 

37. Iodine Number of Linseed. and Petroleum Oils: (17 pp.), 

38. Observations on Finishing Temperatures and Properties of Rails (63 pp.). 

39. Analysisof Printing Inks (20 pp.). 

40. The Veritas Firing Rings (10 pp.). 

41. Lead Acetate Test for Hydrogen Sulphide in Gas (46 pp.), 

42. Standardization of No. 200 Cement Sieves (51 pp.). 

43. Hydration of Portland Cement (71 pp.). 

44. Investigation of the Durability of Cement Drain Tile in Alkali Soils (56 pp.). 

45 A Study of Some Recent Methods for the Determination of Total Sulphur in 
Rubber (16 pp.), 

46. i^ Study of the Atterberg Plasticity Method (18 pp .) . 

47. The Value of the High Pressure of Steam Test of Portland Cement (34 pp.). 



An Air Analyzer for Determining the Fineness of Cement (74 pp.). 

Emergent Stem Correction for Thermometers in Creosote Oil Distillation Flasks 

(21pp.). 
Viscosity of Porcelain Bodies High in Feldspar (7 pp.). 

Use of Sodium Salts in the Purification of Clays and in the Casting Process (40 pp. ). 
Electrolysis and Its Mitigation (143 pp.). 
An Investigation of Fusible Tin Boiler Plugs (37 pp.). 
Special Studies in Electrolysis Mitigation. III. A Report on Conditions in 

Springfield, Ohio, with Insulated Feeder System Installed (64 pp.). 

55. Special Studies in Electrolysis Mitigation. XV. A Preliminary Study of Elec- 

trolysis Conditions in Elyria, Ohio, with Recommendations for Mitigation 

(49 PP-)- 

56. Protection of Life and Property Against Lightning (127 pp.)- 

57. Difference in Weight Between Raw and Clean Wools (— pp.). 

58. Compressive. Strength of Portland Cement Mortars and Concretes (— pp.)* 

59. Standard Test Specimens of Zinc Bronze (Cu 88, Sn 10, Zn 2). 

Part I. Preparation of Specifications. Part II. Microstructure (— pp.). 
[NoTS.— A complete list of Circulars, Scientific Papers, and miscellaneous publi- 
cations may be obtained free of charge on application to the Bureau of Standards, 
Washington, 0. C] 



